Propellers.—To turn now to the propellers. Unless the reader has already had fair experience in making model propellers, he should purchase a couple, one right-handed and one left-handed, as they have to revolve in opposite directions. It would be quite impossible to give in the compass of this article such directions as would enable a novice to make a really efficient propeller, and it must be efficient for even a decent flight with a self-launching model. The diameter of the two propellers should be about 11-1/2 to 11-3/4 inches, with a pitch angle at the extremities of about 25 to 30 degrees as a limit. The "centrale" type (Fig. 135) is to be preferred. Such propellers can be procured at Messrs. A. W. Gamage, Ltd., Holborn, E.C.; Messrs. T. W. K. Clarke and Co., Kingston-on-Thames; and elsewhere.

For the particular machine which we are considering, the total weight of the two propellers, including axle and hook for holding the rubber, should not exceed 3/4 oz. This means considerable labour in cutting and sandpapering away part of the boss, which is always made much too large in propellers of this size. It is wonderful what can be done by care and patience. The writer has in more than one case reduced the weight of a propeller by more than one-half by such means, and has yet left sufficient strength.

The combined axle and hook should be made as follows:—Take a piece of thin steel wire, sharpen one end, and bend it as shown at C (Fig. 136). Pass the end B through a tight-fitting hole in the centre of the small boss of the propeller, and drive C into the wood. Solder a tiny piece of 1/8-inch brass tubing to the wire axle at A, close up to the rubber hook side of the propeller, and file quite smooth. The only things now left to do are to bend the wire into the form of a hook (as shown by the dotted line), and to cover this hook, as already advised, with a piece of valve tubing to prevent fraying the rubber skeins.



Weight.—The weight of a model with a T-shaped central rod 1/16 inch thick should be 4-1/2 oz. Probably it will be more than this—as a maximum let us fix 6 oz.—although 4-1/2 oz. is quite possible, as the writer has proved in actual practice. In any case the centre of gravity of the machine without the rubber motor should be situated 1 inch behind the front or entering edge of the main plane. When the rubber motor (14 strands of 1/16-inch rubber for each propeller, total weight 2 oz.) is in position, the centre of gravity will be further forward, in front of the main plane. The amount of rubber mentioned is for a total weight of 6-1/2 oz. If the weight of the model alone be 6 oz., you will probably have to use 16 strands, which again adds to the weight, and makes one travel in a vicious circle. Therefore I lay emphasis on the advice, Keep down the weight.

The front edge of the elevator should be set about 3/8 inch higher than the back, and the model be tried first as a glider, with the rubber and propellers in position. If it glides satisfactorily, wind up the motor, say 500 turns, and launch by hand. When a good flight has been obtained, and the correct angle of the elevator has been determined, place the model on a strip of linoleum, wind up, and release the propellers. The model should rise in its own length and remain in the air (if wound up 900 turns) at least three quarters of a minute. Choose a calm day if possible. If a wind blows, let the model face the breeze. Remember that the model flies high, and select a wide open space. Do not push the model forward; just release the propellers, held one in each hand near the boss by the fingers and thumb. As a lubricant for the rubber use pure glycerine. It is advisable to employ a geared-up mechanical winder, since to make 1,800 turns with the fingers is rather fatiguing and very tedious.

Simple as this model may seem in design, one built by the writer on exactly the lines given has met the most famous flying models of the day in open competition and proved successful against them.



XXVI. APPARATUS FOR SIMPLE SCIENTIFIC EXPERIMENTS.

Colour Discs for the Gramophone.—The gramophone, by virtue of its table revolving at a controllable speed, comes in useful for a series of optical experiments made with coloured discs bearing designs of different kinds.

The material needed for these discs is cardboard, covered with white paper on one side, or the Bristol board used by artists. The discs on which the designs are drawn should be made as large as the gramophone table will take conveniently, so as to be viewed by a number of people at once. To encourage readers who do not possess a gramophone, it may be pointed out that a gramophone, is merely a convenience, and not indispensable for turning the discs, which may be revolved on a sharpened pencil or any other spindle with pointed ends.

The Vanishing Spirals (Fig. 137).—This design, if spun slowly in a clockwise direction, gives one the impression that the lines all move in towards the centre. If the disc is turned in an anti-clockwise direction, the lines seem to move towards the circumference and disappear. To get the proper effect the gaze should be fixed and not attempt to follow the lines round.



The Rolling Circles.—Figs. 138 and 139 are variations of the same idea. In Fig. 138 two large circles are described cutting one another and enclosing a smaller circle concentric with the disc. When spun at a certain rate the larger circles will appear to run independently round the small. The effect is heightened if the circles are given different colours. If black only is used for the large circles, the eyes should be kept half closed. In Fig. 139 two pairs of circles are described about two centres, neither of which is the centre of the disc. The pairs appear to roll independently.



The Wriggling Line (Fig. 140).—If this design is revolved at a low speed and the eye is fixed on a point, the white (or coloured) line will seem to undulate in a very extraordinary manner. The line is made up of arcs of circles, and as the marking out is somewhat of a geometrical problem, a diagram (Fig. 141) is added to show how it is done. The dotted curves are those parts of the circles which do not enter into the design.

Begin by marking out the big circle A for the disc. The circumference of this is divided into six equal parts (chord equal to radius), and through the points of division are drawn the six lines from the centre. Describe circles aaa, each half the diameter of A. The circles bbb are then drawn from centres on the lines RRR, and with the same radius as aaa., The same centres are used for describing the circles a1 a1 a1 and b1 b1 b1, parts of which form the inner boundary of the line. The background should be blackened and the belt left white or be painted some bright colour.



Another optical illusion is afforded by Fig. 142. Two sets of circles are described about different centres, and the crescent-shaped areas between them coloured, the remainder of the disc being left white. The disc is revolved about the centre of the white areas, and one gets the impression that the coloured parts are portions of separate discs separated by white discs.



The Magic Spokes (Fig. 143).—Place a design like this on the gramophone and let it turn at high speed. The radial lines seem but a blur. Now punch a hole one-eighth of an inch in diameter in a piece of blackened card, and, standing well away from the gramophone, apply your eye to the hole and move the card quickly to and fro. The extreme briefness of the glimpses obtained of the moving lines seems to rob them of motion, or even make them appear to be moving in the direction contrary to the actual. Instead of a single hole, one may use a number of holes punched at equal intervals round a circle, and revolve the card on the centre. If a certain speed be maintained, the spokes will appear motionless.

The substitution of a long narrow slit for a circular hole gives other effects.



A Colour Top.—Cut a 4-inch disc out of white cardboard and blacken one-half with Indian ink. On the other half draw four series of concentric black lines, as shown in Fig. 144. If the disc is mounted on a knitting needle and spun in a horizontal plane, the black lines will appear of different colours. A clockwise rotation makes the outermost lines appear a greenish blue, those nearest the centre a dark red, and the intermediate groups yellow and green. A reversal of the motion reverses the order of the colours, the red lines now being farthest from the centre. The experiment is generally most successful by artificial light, which contains a larger proportion of red and yellow rays than does sunlight. The speed at which the top revolves affects the result considerably. It should be kept moderate, any excess tending to neutralize the colours.



The Magic Windmill.—Mark a circle 2-1/2 inches in diameter on a piece of notepaper, resting the centre leg [of the compass] so lightly that it dents without piercing the paper. With the same centre describe a 3/4-inch circle. Join the circles by eight equally spaced radial lines, and an eighth of an inch away draw dotted parallel lines, all on the same side of their fellow lines in order of rotation. Cut out along the large circle, and then with a. sharp knife follow the lines shown double in Fig. 145. This gives eight little vanes, each of which must be bent upwards to approximately the same angle round a flat ruler held with an edge on the dotted line. Next make a dent with a lead pencil at the exact centre on the vane side, and revolve the pencil until the dent is well polished.



Hold a pin, point upwards, in the right hand, and with the left centre the mill, vanes pointing downwards, on the pin (Fig. 146). The mill will immediately commence to revolve at a steady pace, and will continue to do so indefinitely; though, if the head of the pin be stuck in, say, a piece of bread, no motion will occur. The secret is that the heat of the hand causes a very slight upward current of warmed air, which is sufficient to make the very delicately poised windmill revolve.

A Pneumatic Puzzle.—For the very simple apparatus illustrated by Fig. 147 one needs only half a cotton reel, three pins, and a piece of glass or metal tubing which fits the hole in the reel. Adjust a halfpenny centrally over the hole and stick the pins into the reel at three equidistant points, so that they do not quite touch the coin, and with their ends sloping slightly outwards to allow the halfpenny to fall away.



Press the coin against the reel and blow hard through the tube. One would expect the coin to fall; but, on the contrary, the harder you blow the tighter will it stick, even if the reel be pointed downwards. Only when you stop blowing will it fall to the floor.

This is a very interesting experiment, and will mystify onlookers who do not understand the reason for the apparent paradox, which is this. The air blown through the reel strikes a very limited part of the nearer side of the halfpenny. In order to escape, it has to make a right-angle turn and pass between coin and reel, and, while travelling in this direction, loses most of its repulsive force. The result is that the total pressure on the underside of the coin, plus the effect of gravity, is exactly balanced by the atmospheric pressure on the outside, and the coin remains at that distance from the reel which gives equilibrium of forces. When one stops blowing, the air pressure on both sides is the same, and gravity makes the coin fall away.

The function of the pins is merely to keep the halfpenny centred on the hole. If steam is used instead of human breath, a considerable weight may be hung from the disc without dislodging it.

The Magic Swingers.—The easily made toy illustrated next is much more interesting than would appear from the mere picture, as it demonstrates a very striking physical phenomenon, the transference of energy. If two pendulums are hung close together from a flexible support and swung, their movements influence one another in a somewhat remarkable way—the swing of the one increasing as that of the other dies down, until a certain point is reached, after which the process is reversed, and the "dying" or "dead" pendulum commences to come to life again at the expense of the other. This alternation is repeated over and over again, until all the energy of both pendulums is exhausted.



To make the experiment more attractive, we substitute for the simplest possible pendulums—weights at the end of strings—small swings, each containing a figure sitting or standing on a seat, to the underside of which is attached a quarter of a pound of lead. To prevent the swings twisting, they are best made of strong wire bent as shown in Fig. 148, care being taken that the sides are of equal length, so that both hooks may press equally on the strings. Eighteen inches is a good length. The longer the swing, and the heavier the weight, the longer will the experiment last.

The swings are hung, six inches apart, from a stout string stretched tightly between two well-weighted chairs or between two fixed points. The string should be at least 4 feet long.

With two equally long and equally weighted pendulums, the three following experiments may be carried out:—

1. Let one, A, start from rest. The other, B will gradually die, and A swing to and fro more and more violently, till B at last comes to a dead stop. Then A will die and B in turn get up speed. The energy originally imparted to B is thus transferred through the string from one pendulum to the other an indefinite number of times, with a slight loss at every alternation, until it is finally exhausted by friction.

2. Swing them in opposite directions, but start A from a higher point than B. They will each alternately lose and gain motion, but will never come to rest, and will continue to swing in opposite directions—that is, while A swings north or east B will be swinging south or west, and vice versa.

3. Start them both in the same direction, but one from a higher point than the other. There will be the same transference of energy as in (2), but neither will come to rest between alternations, and they will always swing in the same direction.

Unequal Lengths.—If for one of the original pendulums we substitute one a couple of inches longer than the other, but of the same weight, the same set of three experiments will provide six variations among them, as in each case either the longer or the shorter may be started first or given the longer initial swing, as the case may be. The results are interesting throughout, and should be noted.

Three or more Pendulums.—If the number of pendulums be increased to three or more, the length of all being the same, a fresh field for observation is opened. With an increase of number a decrease in the individual weighting is advisable, to prevent an undue sagging of the string.

In conclusion, we may remark that a strong chain stretched between two trees and a suitable supply of rope will enable the reader and his friends to carry out all the experiments on a life-size scale.

A Smoke-ring Apparatus.—Get a large tin of the self-opening kind and cut a hole 2 inches across in the bottom. Then make a neat circular hole 1-1/4 inches in diameter in the centre of a paper disc somewhat smaller than the bottom of the tin, to which it is pasted firmly on the outside. The other end—from which the lid is removed—must be covered with a piece of sheet rubber stretched fairly tight and secured to the tin by string passed over it behind the rim. An old cycle or motor car air tube, according to the size of the tin, will furnish the rubber needed; but new material, will cost only a few pence (Fig. 149).



A dense smoke is produced by putting in the tin two small rolls of blotting paper, one soaked in hydrochloric acid, the other in strong ammonia. The rolls should not touch. To reduce corrosion of the tin by the acid, the inside should be lined with thin card.



A ring of smoke is projected from the hole in the card if the rubber diaphragm is pushed inwards. A slow, steady push makes a fat, lazy ring come out; a smart tap a thinner one, moving much faster. Absolutely still air is needed for the best effects, as draughts make the rings lose shape very quickly and move erratically. Given good conditions, a lot of fun can be got out of the rings by shooting one through another which has expanded somewhat, or by destroying one by striking it with another, or by extinguishing a candle set up at a distance, and so on. The experimenter should notice how a vortex ring rotates in itself while moving forward, like a rubber ring being rolled along a stick.

A continuous supply of smoke can be provided by the apparatus shown in Fig. 150. The bulb of a scent spray is needed to force ammonia gas through a box, made air-tight by a rubber band round the lid, in which is a pad soaked with hydrochloric acid. The smoke formed in this box is expelled through a pipe into the ring-making box.

Caution.—When dealing with hydrochloric acid, take great care not to get it on your skin or clothes, as it is a very strong corrosive.



XXVII. A RAIN-GAUGE.

The systematic measurement of rainfall is one of those pursuits which prove more interesting in the doing than in the prospect. It enables us to compare one season or one year with another; tells us what the weather has been while we slept; affords a little mild excitement when thunderstorms are about; and compensates to a limited extent for the disadvantages of a wet day.

The general practice is to examine the gauge daily (say at 10 a.m.); to measure the water, if any, collected during the previous twenty-four hours; and to enter the record at once. Gauges are made which record automatically the rainfall on a chart or dial, but these are necessarily much more expensive than those which merely catch the water for measurement.

This last class, to which our attention will be confined chiefly, all include two principal parts—a metal receiver and a graduated glass measure, of much smaller diameter than the receiver, so that the divisions representing hundredths of an inch may be far enough apart to be distinguishable. It is evident that the smaller the area of the measure is, relatively to that of the receiver, the more widely spaced will the graduation marks of the measure be, and the more exact the readings obtained.



The gauge most commonly used is that shown in Fig. 151. It consists of an upper cylindrical part, usually 5 or 8 inches in diameter, at the inside of the rim, with its bottom closed by a funnel. The lower cylindrical part holds a glass catcher into which the funnel delivers the water for storage until the time when it will be measured in a graduated glass. The upper part makes a good fit with the lower, in order to reduce evaporation to a minimum.

Such a gauge can be bought for half a guinea or so, but one which, if carefully made, will prove approximately accurate, can be constructed at very small expense. One needs, in the first place, a cylindrical tin, or, better still, a piece of brass tubing, about 5 inches high and not less than 3 inches in diameter. (Experiments have proved that the larger the area of the receiver the more accurate are the results.) The second requisite is a piece of stout glass tubing having an internal diameter not more than one-quarter that of the receiver This is to serve as measuring glass.



The success of the gauge depends entirely upon ascertaining accurately how much of the tube will be filled by a column of water 1 inch deep and having the same area as the receiver. This is easily determined as follows:—If a tin is to be used as receiver, make the bottom and side joints watertight with solder; if a tube, square off one end and solder a flat metal to it temporarily. The receptacle is placed on a perfectly level base, and water is poured in until it reaches exactly to a mark made 4 inches from the end of a fine wire held perpendicularly. Now cork one end of the tube and pour in the water, being careful not to spill any, emptying and filling again if necessary. This will give you the number of tube inches filled by the 4 inches in the receiver. Divide the result by 4, and you will have the depth unit in the measure representing 1 inch of rainfall. The measuring should be done several times over, and the average result taken as the standard. If the readings all agree, so much the better.

Preparing the Scale.—The next thing is to graduate a scale, which will most conveniently be established in indelible pencil on a carefully smoothed strip of white wood 1 inch wide. First make a zero mark squarely across the strip near the bottom, and at the unit distance above it a similar mark, over which "One Inch" should be written plainly. The distance between the marks is next divided by 1/2-inch lines into tenths, and these tenths by 1/4-inch lines into hundredths, which, if the diameter of the receiver is four times that of the tube, will be about 3/16 inch apart. For reading, the scale is held against the tube, with the zero mark level with the top of the cork plugging the bottom. It will, save time and trouble if both tube and scale are attached permanently to a board, which will also serve to protect the tube against damage.

Making the Receiver.—A tin funnel, fitting the inside of the receiver closely, should be obtained, or, if the exact article is not available, a longer one should be cut down to fit. Make a central hole in the bottom of the receiver large enough to allow the funnel to pass through up to the swell, and solder the rim of the funnel to the inside of the receiver, using as little heat as possible.

If you select a tin of the self-opening kind, you must now cut away the top with a file or hack-saw, being very careful not to bend the metal, as distortion, by altering the area of the upper end of the tin, will render the gauge inaccurate.

The receiver should be supported by another tin of somewhat smaller diameter, and deep enough to contain a bottle which will hold 3 or 4 inches of rainfall. In order to prevent water entering this compartment, tie a strip of rubber (cut out of an old cycle air tube) or other material round the receiver, and projecting half an inch beyond the bottom (Fig. 152).

All tinned iron surfaces should be given a couple of thin coats or paint.

The standard distance between the rain gauge and the ground is one foot. The amount caught decreases with increase of elevation, owing to the greater effect of the wind. The top of the gauge must be perfectly level, so that it may offer the same catchment area to rain from whatever direction it may come.



Another Arrangement.—To simplify measurement, the receiver and tube may be arranged as shown in Fig. 153. In this case the water is delivered directly into the measure, and the rainfall may be read at a glance. On the top of the support is a small platform for the receiver, its centre directly over the tube. The graduations, first made on a rod as already described, may be transferred, by means of a fine camel's hair brush and white paint, to the tube itself. To draw off the water after taking a reading, a hole should be burnt with a hot wire through the bottom cork. This hole is plugged with a piece of slightly tapered brass rod, pushed in till its top is flush with the upper surface of the cork.

If the tube has small capacity, provision should be made for catching the overflow by inserting through the cork a small tube reaching to a convenient height-say the 1-inch mark. The bottom of the tube projects into a closed storage vessel. Note that the tube must be in position before the graduation is determined, otherwise the readings will exaggerate the rainfall.



Protection against the Weather.—A rain-gauge of this kind requires protection against frost, as the freezing of the water would burst the tube. It will be sufficient to hinge to the front of the support a piece of wood half an inch thicker than the diameter of the tube, grooved out so as to fit the tube when shut round it (Fig 154).



XXVIII. WIND VANES WITH DIALS.

It is difficult to tell from a distance in which direction the arrow of a wind vane points when the arrow lies obliquely to the spectator, or points directly towards or away from him. In the case of a vane set up in some position where it will be plainly visible from the house, this difficulty is overcome by making the wind vane operate an arrow moving round a vertical dial set square to the point of observation. Figs. 155 to 157 are sketches and diagrams of an apparatus which does the work very satisfactorily. The vane is attached to the upper end of a long rod, revolving freely in brackets attached to the side of a pole. The bottom end of the rod is pointed to engage with a nick in a bearer, in which it moves with but little friction. Near the end is fixed a horizontal bevel-wheel, engaging with a vertical bevel of equal size and number of teeth attached to a short rod running through a hole in the post to an arrow on the other side. Between arrow and post is room for a dial on which the points of the compass are marked.

The construction of the apparatus is so simple as to call for little comment. The tail of the vane is made of two pieces of zinc, tapering from 8 inches wide at the rear to 4 inches at the rod, to which they are clipped by 4 screws and nuts. A stay soldered between them near the stern keeps the broader ends a couple of inches apart, giving to the vane a wedge shape which is more sensitive to the wind than a single flat plate. The pointer also is cut out of sheet metal, and is attached to the tail by means of the screws already mentioned. It must, of course, be arranged to lie in a line bisecting the angle formed by the two parts of the tail.



The rod should preferably be of brass, which does not corrode like iron. If the uppermost 18 inches or so are of 1/4-inch diameter, and assigned a bracket some distance below the one projecting from the top of the pole, the remainder of the rod need not exceed 1/8 to 5/32 inch in diameter, as the twisting strain on it is small. Or the rod may be built up of wooden rods, well painted, alternating with brass at the points where the brackets are.



The Bevel Gearing.—Two brass bevel wheels, about 1 inch in diameter, and purchasable for a couple of shillings or less, should be obtained to transmit the vane movements to the dial arrow. Grooved pulleys, and a belt would do the work, but not so positively, and any slipping would, of course, render the dial readings incorrect. The arrow spindle (of brass) turns in a brass tube, driven tightly into a hole of suitable size bored through the centre of the post (Fig. 157). It will be well to fix a little metal screen over the bevel gear to protect it from the weather.



The Dial—This is made of tinned iron sheet or of 1/4-inch wood nailed to 1/2-inch battens. It is held up to the post by 3-inch screws passing through front and battens. At the points of contact, the pole is slightly flattened to give a good bearing; and, to prevent the dial being twisted off by the wind, strip iron or stout galvanized wire stays run from one end of a batten to the other behind the post, to which they are secured.

The post should be well painted, the top protected by a zinc disc laid under the top bracket, and the bottom, up to a point 6 inches above the ground level, protected by charring or by a coat of boiled tar, before the dial and the brackets for the vane rod to turn in are fastened on. A white dial and black arrow and letters will be most satisfactory against a dark background; and vice versa for a light background. The letters are of relatively little importance, as the position of the arrow will be sufficient indication.

It gives little trouble to affix to the top of the pole 4 arms, each carrying the initial of one of the cardinal points of the compass. The position of these relatively to the direction in which the dial will face must be carefully thought out before setting the position in the ground. In any case the help of a compass will be needed to decide which is the north.

Having set in the post and rammed the earth tightly round it, loosen the bracket supporting the vane rod so that the vane bevel clears the dial bevel. Turn the vane to true north, set the dial arrow also to north, and raise the bevel so that it meshes, and make the bracket tight.

Note.—In the vicinity of London true north is 15 degrees east of the magnetic north.

The pole must be long enough to raise the vane clear of any objects which might act as screens, and its length will therefore depend on its position. As for the height of the dial above the ground, this must be left to individual preference or to circumstances. If conditions allow, it should be near enough to the ground to be examined easily with a lamp at night, as one of the chief advantages of the system is that the reading is independent of the visibility of the vane.

A Dial Indoors.—If some prominent part of the house, such as a chimney stack, be used to support the pole—which in such a case can be quite short—it is an easy matter to connect the vane with a dial indoors, provided that the rod can be run down an outside wall.

An Electrically Operated Dial.—Thanks to the electric current, it is possible to cause a wind vane, wherever it may be set, to work a dial situated anywhere indoors. A suggested method of effecting this is illustrated in Figs. 158 to 161, which are sufficiently explicit to enable the reader to fill in details for himself.



In-this case the vane is attached (Fig. 158) to a brass tube, closed at the upper end, and supported by a long spike stuck into the top of the pole. A little platform carries a brass ring, divided into as many insulated segments as the points which the vane is to be able to register. Thus, there will be eight segments if the half-points as well as the cardinal points are to be shown on the dial. The centre of each of these segments lies on a line running through the centre of the spike to the compass point to which the segment belongs. The tube moves with it a rotating contact piece, which rubs against the tops of the segments.

Below it is a "brush" of strip brass pressing against the tube. This brush is connected with a wire running to one terminal of a battery near the dial.



The Dial.—This may be either vertical or horizontal, provided that the arrow is well balanced. The arrow, which should be of some light non-magnetic material, such as cardboard or wood, carries on its lower side, near the point, a piece of soft iron. Under the path of this piece is a ring of equally spaced magnets, their number equaling that of, the segments on the vane. Between arrow and magnets is the dial on which the points are marked (Fig. 159).

Each segment is connected by a separate wire with the corresponding dial magnet, and each of these, through a common wire and switch, with the other terminal of the battery (Fig. 161).

In order to ascertain the quarter of the wind, the switch is closed. The magnet which is energized will attract the needle to it, showing in what direction the vane is pointing. To prevent misreading, the dial may be covered by a flap the raising of which closes the battery circuit. A spring should be arranged to close the flap when the hand is removed, to prevent waste of current.



The exactitude of the indication given by the arrow depends on the number of vane segments used. If these are only four, a N. read- ing will be given by any position of the vane between N.E. and N.W.; if eight, N. will mean anything between N.N.E. and N.N.W. Telephone cables, containing any desired number of insulated wires, each covered by a braiding of a distinctive colour, can be obtained at a cost only slightly exceeding that of an equal total amount of single insulated wire. The cable form is to be preferred, on account of its greater convenience in fixing.

The amount of battery power required depends on the length of the circuit and the delicacy of the dial. If an ordinary compass needle be used, as indicated in Fig. 160, very little current is needed. In this case the magnets, which can be made of a couple of dozen turns of fine insulated wire round a 1/8-in soft iron bar, should be arranged spokewise round the compass case, and care must be taken that all the cores are wound in the same direction, so as to have the same polarity. Otherwise some will attract the N. end of the needle and others repel it. The direction of the current flow through the circuit will decide the polarity of the magnets, so that, if one end of the needle be furnished with a little paper arrow-head, the "correspondence" between vane and dial is easily established. An advantage attaching to the use of a compass needle is that the magnet repels the wrong end of the needle.



The brush and segments must be protected from he weather by a cover, either attached to the segment platform or to the tube on which the vane is mounted.

The spaces between the segments must be filled in flush with some non-conducting material, such as fibre, vulcanite, or sealing-wax; and be very slightly wider than the end of the contact arm, so that two segments may not be in circuit simultaneously. In certain positions of the vane no contact will be made, but, as the vane is motionless only when there is no wind or none to speak of, this is a small matter.



XXIX. A STRENGTH-TESTING MACHINE.

The penny-in-the-slot strength-testing machine is popular among men and boys, presumably because many of them like to show other people what their muscles are capable of, and the opportunity of proving it on a graduated dial is therefore tempting, especially if there be a possibility of recovering the penny by an unusually good performance.

For the expenditure of quite a small number of pence, one may construct a machine which will show fairly accurately what is the value of one's grip and the twisting, power of the arms; and, even if inaccurate, will serve for competitive purposes. The apparatus is very simple in principle, consisting of but five pieces of wood, an ordinary spring balance registering up to 40 lbs., and a couple of handles. The total cost is but a couple of shillings at the outside.

Fig. 162 is a plan of the machine as used for grip measuring. The base is a piece of deal 1 inch thick, 2 feet long, and 5-1/2 inches wide. The lever, L, is pivoted at P, attached to a spring balance at Q, and subjected to the pull of the hand at a point, R.

The pressure exerted at R is to that registered at Q as the distance PQ is to the distance PR. As the spring balance will not record beyond 40 lbs., the ratio of PQ to PR may conveniently be made 5 to 1, as this will allow for the performances of quite a strong man; but even if the ratio be lowered to 4 to 1, few readers will stretch the balance to its limit.

The balance should preferably be of the type shown in Fig. 162, having an indicator projecting at right angles to the scale through a slot, as this can be very easily fitted with a sliding index, I, in the form of a 1/4-inch strip of tin bent over at the ends to embrace the edges of the balance.

CONSTRUCTION.



As the pressures on the machine are high, the construction must be solid throughout. The lever frame, A, and pivot piece, C, should be of one-inch oak, and the two last be screwed very securely to the baseboard. The shape of A is shown in Fig. 163. The inside is cut out with a pad saw, a square notch being formed at the back for the lever to move in. The handles of an old rubber chest expander come in useful for the grips. One grip, D, is used entire for attachment to the lever; while of the other only the wooden part is required, to be mounted on a 1/4-inch steel bar running through the arms of A near the ends of the horns. If a handle of this kind is not available for D, one may substitute for it a piece of metal tubing of not less than 1/2-inch diameter, or a 3/4-inch wooden rod, attached to an eye on the lever by a wire passing through its centre.

A handle, if used, is joined to the lever by means of a brass plate 3/4 inch wide and a couple of inches long. A hole is bored in the centre somewhat smaller than the knob to which the rubber was fastened, and joined up to one long edge by a couple of saw cuts. Two holes for good-sized screws must also be drilled and countersunk, and a socket for the knob must be scooped out of the lever. After making screw holes in the proper positions, pass the shank of the knob through the slot in the plate, and screw the plate on the lever. This method holds the handle firmly while allowing it to move freely.

The lever tapers from 1-1/2 inches at the pivot to 5/8 inch at the balance end. The hole for the pivot—5/16-inch steel bar—should be long enough to admit a piece of tubing fitting the bar, to diminish friction, and an important point, be drilled near the handle edge of the lever, so as to leave plenty of wood to take the strain. The last remark also applies to the hole for the balance pin at Q.

The balance support, B, and the pivot piece, C, are 2-1/2 and 2-7/16 inches high respectively. Run a hole vertically through C and the baseboard for the pivot, which should be 4-1/2 inches long, so as to project 1 inch when driven right home. Take some trouble over getting the holes in L and C quite square to the baseboard, as any inaccuracy will make the lever twist as it moves. To prevent the pivot cutting into the wood, screw to the top of C a brass plate bored to fit the pivot accurately. The strain will then be shared by the screws.

The horns of A should be long enough to allow the outside of the fixed grip to be 2-1/4 inches from the inside of the handle.

The balance is secured first to the lever by a pin driven through the eye of the hook, and then to B by a 3-inch screw passed through the ring. The balance should just not be in tension.

When the apparatus is so far complete, test it by means of a second balance applied to D. Set the scale-marker at zero, and pull on the D balance till, say, 35 lbs. is attained. If the fixed balance shows 7 lbs. on what is meant to be a 5 to 1 ratio, the setting of R relatively to P and Q is correct. If, however, there is a serious discrepancy, it would be worth while making tests with a very strong balance, and establishing a corrected gradation on a paper dial pasted to the face of E.

For twisting tests we need a special handle (see Fig. 164), which is slipped on to the pivot and transmits the twist to L through a pin pressing on the back of the lever. The stirrup is made out of strip iron, bent to shape and drilled near the ends for the grip spindle. To the bottom is screwed and soldered a brass or iron plate, into the underside of which the pin is driven.



To prevent the handle bending over, solder round the pivot hole 3/4 inch of brass tubing, fitting the pivot closely.

Tests.—Grip tests should be made with each hand separately. The baseboard should lie flat on a table or other convenient support, and be steadied, but not pushed, by the hand not gripping.

Twisting tests may be made inwards with the right hand, and back-handedly with the left. The apparatus is stood on edge, square to the performer, resting on the horns of A and a support near the balance.

Finger tests are made by placing the thumb on the front face of B, and two fingers on the farther side of the lever, one to the left and the other to the right of the tail of the balance.



XXX. LUNG-TESTING APPARATUS.

The capacity of the lungs, and their powers of inspiration and expiration, can be tested by means of easily constructed apparatus which will interest most people who are introduced to it. The reduction of the capabilities of the lungs to figures affords a not unprofitable form of entertainment, as even among adults these figures will be found to vary widely.

Air Volume Measuring.—The air which the lungs deal with is scientifically classified under four heads:

1. Tidal air, which passes into and out of the lungs in natural breathing. About 30 cubic inches in an adult (average).

2. Reserve air, which can be expelled after a normal expiration. About 100 cubic inches.

3. Complemental air, which can be drawn in after a normal inspiration. About 100 cubic inches.

4. Residual air, which cannot be removed from the lungs under any conditions by voluntary effort. About 120 cubic inches.

The first three added together give the vital capacity. This, as an addition sum will show, is very much greater than the volume of air taken in during a normal inspiration.

The simplest method of testing the capacity of an individual pair of lungs is embodied in the apparatus shown in Figs. 165 and 166. A metal box is submerged, bottom upwards, in a tank of somewhat larger dimensions, until the water is level with the bottom inside and out. A counterweight is attached to the smaller box to place it almost in equilibrium, so that if air is blown into the box it will at once begin to rise.

If we make the container 7-1/16 inches square inside, in plan, every inch it rises will represent approximately 50 cubic inches of air blown in; and a height of 7 inches, by allowing for 325 cubic inches, with a minimum immersion of half an inch, should suffice even for unusually capacious lungs. The outside box need not be more than 8 inches all ways.



Unless you are an expert with the soldering iron, the making of the boxes should be deputed to a professional tinman, who would turn out the pair for quite a small charge. Specify very thin zinc for the air vessel, and have the top edges stiffened so that they may remain straight.

On receiving the boxes, cut a hole 3/4-inch diameter in the centre of the bottom of the air vessel, and solder round it a piece of tubing, A, 1 inch long, on the outside of the box. In the centre of the larger box make a hole large enough to take a tube, E, with an internal diameter of 1/8 inch. This tube is 8 inches long and must be quite straight. Next procure a straight wire, C, that fits the inside of the small tube easily; make an eye at the end, and cut off about 9 inches. Bore a hole for the wire in a metal disc 1 inch across.



The air container is then placed in the water box and centred by means of wooden wedges driven in lightly at the corners. Push the small tube through its hole in the water box, and thrust the wire—after passing it through the disc and the projection on the air container—into the tube. The tube should reach nearly to the top of the air container, and the wire to the bottom of the water box. Solder the tube to the box, the wire to the disc, and the disc to the container. A little stay, S, will render the tube less liable to bend the bottom of the box. Plug the tube at the bottom.

The wire sliding in the tube will counteract any tendency of the container to tilt over as it rises.

A nozzle, D, for the air tube is soldered into the side of A, as shown.

The counterweight is attached to the container by a piece of fine strong twine which passes over two pulleys, mounted on a crossbar of a frame screwed to the sides of the water box, or to an independent base. The bottom of the central pulley should be eight inches above the top of the container, when that is in its lowest position.

For recording purposes, make a scale of inches and tenths, and the corresponding volumes of air, on the side of the upright next the counterweight. The wire, W, is arranged between counterweight and upright so that an easily sliding plate, P, may be pushed down it by the weight, to act as index.



Notes.—The pulleys must work easily, to reduce friction, which renders the readings inaccurate. Absolute accuracy is not obtainable by this apparatus, as the rising of the container lowers the water level slightly, and the air has to support part of the weight of the container which was previously borne by the water. But the inaccuracy is so small as to be practically negligible.

A Pressure Recorder.

[Transcribers note: Even with the precautions used in this project, health standards of 2004 would consider any exposure to mercury dangerous. Water could be substituted and the column lengths scaled up by about 13.5.]

If mercury is poured into a vertical tube closed at the bottom, a pressure is exerted on the bottom in the proportion of approximately one pound per square inch for every two inches depth of mercury. Thus, if the column is 30 inches high the bottom pressure is slightly under 15 lbs. per square inch.

This fact is utilized in the pressure recorder shown in Fig. 167, a U-shaped glass tube half filled with mercury. A rubber tube is attached to the bent-over end of one of the legs, so that the effects of blowing or suction may be communicated to the mercury in that leg. Normally the mercury stands level in both tubes at what may be called the zero mark. Any change of level in one leg is accompanied by an equal change in the opposite direction in the other. Therefore, if by blowing the mercury is made to rise an inch in the left leg, the pressure exerted is obviously that required to support a two-inch column of mercury—that is, 1 lb. per sq. inch. This gives a very convenient standard of measurement, as every inch rise above the zero mark indicates 1 lb. of pressure.

CONSTRUCTION.

The mercury tube should be made first. Take a piece of glass tubing 20 inches long, and bend it at a point 9 inches from one end after heating in a spirit flame. The legs should be kept as parallel as possible. Lay the tube, while the heated part is still pliant, on a flat surface, the bend projecting over the edge, So that the two legs shall be in line. When the glass has cooled, bend over two inches of the longer leg to an angle of about 45 degrees.

A standard for the tube is now made out of one-inch wood. Hollow out a bed in which the tube shall lie and be completely protected. To the right of the tube the standard is notched to take a small bottle. The notch should be slightly narrower than the diameter of the bottle, and have its sides hollowed out to fit.

Halfway up the tube draw a zero mark across the standards, and above this a scale of inches in fractions on both sides. Each inch represents 1 lb. pressure.

The cork of the bottle must be pierced with a red-hot wire for two glass tubes, one of which is bent over for the blowing tube. Both tubes should be pointed at the bottle end so that they may enter the cork easily. Make the top of the cork air tight with sealing-wax. The purpose of the bottle is to catch any mercury that might be sucked out of the tube; one does not wish mercurial poisoning to result from the experiments. Also it prevents any saliva entering the mercury tube.

When the latter has been secured to the standard by a couple of slips of tin nailed to the front, connect it up with the bottle, and fill it up to the zero mark with mercury poured in through a small paper funnel.

The open end of the tube should be provided with an inch of tubing. Clips placed on this and on the rubber connection between tube and bottle will prevent the escape of mercury should the apparatus be upset when not in use.

The average blowing pressure of which the lungs are capable is about 1-1/2 lbs. per square inch; inspiration pressure without mouth suction about 1 lb. per square inch; suction pressure 2-1/2 to 3 lbs. per square inch.

Caution.—Don't ask people with weak lungs to try experiments with the apparatus described in this chapter.



XXXI. HOME-MADE HARMONOGRAPHS.

Have you ever heard of the harmonograph? If not, or if at the most you have very hazy ideas as to what it is, let me explain. It is an instrument for recording on paper, or on some other suitable surface, the figures described by two or more pendulums acting in concert.

The simplest form of harmonograph is shown in Fig. 168. Two pendulums are so suspended on points that their respective directions of movement are at right angles to one another—that is, pendulum A can swing only north and south, as it were, and pendulum B only east and west. On the top of B is a platform to carry a card, and on the upper end of A a lever is pivoted so as to be able to swing only vertically upwards and downwards. At its end this lever carries a pen, which when at rest lies on the centre of the card platform.



The bob, or weight, of a pendulum can be clamped at any point on its rod, so that the rate or "period" of swing may be adjusted or altered. The nearer the weight is brought to the point of suspension, the oftener will the pendulum swing to and fro in a given time—usually taken as one minute. From this it is obvious that the rates of swing of the two pendulums can be adjusted relatively to one another. If they are exactly equal, they are said to be in unison, and under these conditions the instrument would trace figures varying in outline between the extremes of a straight line on the one hand and a circle on the other. A straight line would result if both pendulums were released at the same time, a circle,[1] if one were released when the other had half finished a swing, and the intermediate ellipses would be produced by various alterations of "phase," or time of the commencement of the swing of one pendulum relatively to the commencement of the swing of the other.

[Footnote 1: It should be pointed out here that the presence of friction reduces the "amplitude," or distance through which a pendulum moves, at every swing; so that a true circle cannot be produced by free swinging pendulums, but only a spiral with coils very close together.]

But the interest of the harmonograph centres round the fact that the periods of the pendulums can be tuned to one another. Thus, if A be set to swing twice while B swings three times, an entirely new series of figures results; and the variety is further increased by altering the respective amplitudes of swing and phase of the pendulums.

We have now gone far enough to be able to point out why the harmonograph is so called. In the case just mentioned the period rates of A and B are as 2: 3. Now, if the note C on the piano be struck the strings give a certain note, because they vibrate a certain number of times per second. Strike the G next above the C, and you get a note resulting from strings vibrating half as many times again per second as did the C strings—that is, the relative rates of vibration of notes C and G are the same as those of pendulums A and B—namely, as 2 is to 3. Hence the "harmony" of the pendulums when so adjusted is known as a "major fifth," the musical chord produced by striking C and G simultaneously.

In like manner if A swings four times to B's five times, you get a "major third;" if five times to B's six times, a "minor third;" and if once to B's three times, a "perfect twelfth;" if thrice to B's five times, a "major sixth;" if once to B's twice, an "octave;" and so on.

So far we have considered the figures obtained by two pendulums swinging in straight lines only. They are beautiful and of infinite variety, and one advantage attaching to this form of harmonograph is, that the same figure can be reproduced exactly an indefinite number of times by releasing the pendulums from the same points.



But a fresh field is opened if for the one-direction suspension of pendulum B we substitute a gimbal, or universal joint, permitting movement in all directions, so that the pendulum is able to describe a more or less circular path. The figures obtained by this simple modification are the results of compounded rectilinear and circular movements.



The reader will probably now see even fresh possibilities if both pendulums are given universal movement. This can be effected with the independent pendulums; but a more convenient method of obtaining equivalent results is presented in the Twin Elliptic Pendulum invented by Mr. Joseph Goold, and shown in Fig. 169. It consists of—(1) a long pendulum, free to swing in all directions, suspended from the ceiling or some other suitable point. The card on which the figure is to be traced, and the weights, are placed on a platform at the bottom of this pendulum. (2) A second and shorter free pendulum, known as the "deflector," hung from the bottom of the first.

This form of harmonograph gives figures of infinite variety and of extreme beauty and complexity. Its chief drawback is its length and weight, which render it more or less of a fixture.

Fortunately, Mr. C. E. Benham of Colchester has devised a Miniature Twin Elliptic Pendulum which possesses the advantages of the Goold, but can be transported easily and set up anywhere. This apparatus is sketched in Fig. 170. The main or platform pendulum resembles in this case that of the Rectilinear Harmonograph, the card platform being above the point of suspension.

Value of the Harmonograph.—A small portable harmonograph will be found to be a good means of entertaining friends at home or elsewhere. The gradual growth of the figure, as the card moves to and fro under the pen, will arouse the interest of the least scientifically inclined person; in fact, the trouble is rather to persuade spectators that they have had enough than to attract their attention. The cards on which designs have been drawn are in great request, so that the pleasure of the entertainment does not end with the mere exhibition. An album filled with picked designs, showing different harmonies and executed in inks of various colours, is a formidable rival to the choicest results of the amateur photographer's skill.

Practical Instructions for making Harmonographs.

Pendulums.—For the Rectilinear type of harmonograph wooden rods 5/8 to 3/4 inch in diameter will be found very suitable. They cost about 2d. each. Be careful to select straight specimens. The upper pendulum of the Miniature Twin Elliptic type should be of stouter stuff, say a broomstick; that of the Goold apparatus stouter still.

All pendulums on which weights are slid up and down should be graduated in inches and fractions, reckoning from the point of suspension as zero. The graduation makes it easy to re-establish any harmony after the weights have been shifted.

Suspensions.—For a harmonograph to give satisfaction it is necessary that very little friction should be set up at the point of suspension, so that the pendulums may lose amplitude of swing very slowly.

One-way suspensions are easily made. Two types, the point and knife-edge respectively, are shown in Fig. 168 and the top part of Fig. 172. The point suspension is most suitable for small rods and moderate weights; the knife-edge for large rods and heavy weights which would tend to crush a fine point.



Points should rest in cup-shaped depressions in a metal plate; knife-edges in V-shaped grooves in a metal ring.



Screws turned or filed to a sharp end make convenient points, as they can be quickly adjusted so that a line joining the points lies exactly at right angles to the pendulum. The cups to take the points should not be drilled until the points have been thus adjusted. Make a punch mark on the bedplate, and using this as centre for one of the points, describe an arc of a circle with the other. This will give the exact centre for the other cup. It is evident that if points and cup centres do not coincide exactly there must be a certain amount of jamming and consequent friction.

In making a knife-edge, such as that shown in Fig. 172, put the finishing touches on with a flat file drawn lengthwise to ensure the edge being rectilinear. For the same reason the V slots in the ring support should be worked out together. If they are formed separately, the chances are against their being in line with one another.

Gimbals, or universal joints, giving motion in all directions, require the employment of a ring which supports one pair of edges or points (Fig. 172), and is itself supported on another pair of edges or points set at right angles to the first. The cups or nicks in the ring should come halfway through, so that all four points of suspension shall be in the same plane. If they are not, the pendulum will not have the same swing-period in all directions. If a gimbal does not work with equal freedom in all ways, there will be a tendency for the pendulum to lose motion in the direction in which most friction occurs.

By wedging up the ring of a gimbal the motion of the pendulum is changed from universal to rectilinear. If you are making a harmonograph of the type shown in Fig. 168, use a gimbal for the platform pendulum, and design it so that the upper suspension gives a motion at right angles to the pen pendulum. The use of two little wedges will then convert the apparatus in a moment from semirectilinear to purely rectilinear.

Weights.—The provision of weights which can be slipped up and down a rod may present some difficulty. Of iron and lead, lead is the more convenient material, as occupying less space, weight for weight, and being more easily cast or shaped. I have found thin sheet roofing lead, running 2 lbs. to the square foot, very suitable for making weights, by rolling a carefully squared strip of the material round the rod on which it will have to move, or round a piece of brass tubing which fits the rod. When the weight has been rolled, drill four holes in it, on opposite sides near the ends, to take nails, shortened so that they just penetrate all the laps but do not enter the central circular space. These will prevent the laps sliding over one another endways. A few turns of wire round the weight over the heads makes everything snug.

Just one caution here. The outside lap of lead should finish at the point on the circumference where the first lap began, for the weight to be approximately symmetrical about the centre.

An alternative method is to melt up scrap lead and cast weights in tins or flowerpots sunk in sand, using an accurately centred stick as the core. This stick should be very slightly larger than the pendulum rod, to allow for the charring away of the outside by the molten metal. (Caution.—The mould must be quite dry.)

Failing lead, tin canisters filled with metal scrap may be made to serve. It will in this case be necessary to bore the lid and bottom centrally and solder in a tube fitting the rod, and to make an opening through which the weighting material can be inserted.

Adjustment of Weights.—As lead is too soft a metal to give a satisfactory purchase to a screw—a thread cut in it soon wears out—it is better to support a leaden weight from underneath by means of a brass collar and screw. A collar is easily made out of a bit of tubing thickened at the point where the screw will pass by soldering on a suitably shaped piece of metal. Drill through the reinforcement and tubing and tap to suit the screw used, which may well be a camera tail screw, with a large flat head.

I experienced some trouble from the crushing of wooden rods by a screw, but got over it as follows. The tubing selected for the collar was large enough to allow a piece of slightly smaller tubing to be introduced between it and the rod. This inner piece was slit from one end almost to the other, on opposite sides, and soldered at one end to the outer tube, a line joining the slots being at right angles to the axis of the screw. The pressure of the screw point was thus distributed over a sufficient area of the wood to prevent indentation. (See Fig. 173.)



Pen Levers.—The pen lever, of whatever kind it be, must work on its pivots with very little friction, and be capable of fine adjustment as regards balance. For the Rectilinear Harmonograph the form of lever pivot shown in Fig. 174 is very suitable. The spindle is a wire nail or piece of knitting needle sharpened at both ends; the bearings, two screws filed flat at the ends and notched with a drill.

The brass standard should be drilled and tapped to fit the screws fairly tight, so that when once adjusted they may not slacken off. If the lever is made of wood, the tail may be provided with a number of metal pegs on which to place the weights; if of wire, the tail should be threaded so that a brass weight and lock screw may be moved along it to any desired position. It is very important that the pressure of the pen on the card should be reduced to a minimum by proper balancing, as the friction generated by a "heavy" pen slows the pendulum very quickly; and that the centre of gravity should be below the point of suspension, to put the pen in stable equilibrium. The lever shown in Fig. 169 is suitable for the Twin Elliptic Pendulum.

In this case the lever is not moved about as a whole. Mr. C. E. Benham advocates the use of wood covered with velvet to rest the lever points on.

For keeping the pen, when not in use, off the platform, a small weight attached to the lever by a thread is convenient. When the pen is working, the weight is raised to slacken the thread.



Attaching Pen to Lever.—In the case of wooden levers, it is sufficient to slit the end centrally for a few inches after drilling a hole rather smaller than the pen, at a point which lies over the centre of the card platform, and quite squarely to the lever in all directions, so that the pen point may rest squarely on the card. (Fig. 175.)

Another method is to attach to the end of the lever a vertical half-tube of tin, against which the pen is pressed by small rubber bands; but even more convenient is a small spring clip shaped as in Fig. 176.



The card platform should be perfectly flat. This is essential for the production of good diagrams. If wood is used, it is advisable to glue two thin pieces together under pressure, with the grain of one running at right angles to the other, to prevent warping.

Another important point is to have the card platform square to the rod. If a piece of tubing fitting the rod is turned up true in the lathe and soldered to a disc screwed to the underside of the table, perpendicularity will be assured, and incidentally the table is rendered detachable.

To hold the card in place on the table, slit a spring of an old photographic printing frame down the middle, and screw the two halves, convex side upwards, by one end near two opposite corners of the platform. (See Fig. 170.) If cards of the same size are always used, the table should be marked to assist adjustment.

Making Pens.—The most satisfactory form of pen is undoubtedly a piece of glass tubing drawn out to a point, which is ground down quite smooth. The making of such pens is rather a tedious business, but if care be taken of the pen when made it will last an indefinite time.

Tubing 3/16 or 1/8 inch in external diameter is suitable. Break it up (by nicking with a file) into 9-inch lengths. Take a piece and hold its centre in the flame of a small spirit lamp, and revolve it till it softens. Then draw the glass out in as straight a line as possible, so that the points may be central. If the drawing is done too fast, the points will be much too long to be of any use: half an inch of taper is quite enough.

Assuming that a point of satisfactory shape has been attained—and one must expect some failures before this happens—the pen may be placed in the pen lever and ground down on a perfectly clean wet hone laid on the card platform, which should be given a circular movement. Weight the lever so as to put a fair pressure on the point.

The point should be examined from time to time under a strong magnifying-glass, and tested by blowing through it into a glass of water. For very liquid ink the hole should be as small as you can possibly get it; thick inks, such as Indian, require coarser pens.

The sharp edge is taken off and the width of the point reduced by drawing the pen at an angle along the stone, revolving it all the time. The nearer to the hole you can wear the glass away the finer will be the line made by the pen.

Another method is as follows:—Seal the point by holding it a moment in the flame. A tiny bulb forms on the end, and this has to be ground away till the central hole is reached. This is ascertained by the water test, or by holding the pen point upwards, so that light is reflected from the tip, and examining it under the magnifier. Then grind the edge off, as in the first case.

Care of Pens.—The ink should be well strained, to remove the smallest particles of "suspended matter," and be kept corked. Fill the pen by suction. On no account allow the ink to dry in the pen. Squirt any ink out of it when it is done with, and place it point downwards in a vessel of water, which should have a soft rubber pad at the bottom, and be kept covered to exclude dust. Or the pen may be cleaned out with water and slipped into a holder made by rolling up a piece of corrugated packing-paper. If the point gets stopped up, stand the pen in nitric or sulphuric acid, which will probably dissolve the obstruction; and afterwards wash it out.

Inks.—I have found Stephens's coloured inks very satisfactory, and can recommend them.

Paper and Cards.—The paper or cards used to draw the figures on should not have a coated surface, as the coating tends to clog the pen. The cheapest suitable material is hot pressed paper, a few penny-worths of which will suffice for many designs. Plain white cards with a good surface can be bought for from 8s. to 10s. per thousand.

Lantern Slides.—Moisten one side of a clean lantern slide plate with paraffin and hold it over a candle flame till it is a dead black all over. Very fine tracings can be obtained on the smoked surface if a fine steel point is substituted for the glass pen. The design should be protected by a cover-glass attached to it by a binding strip round the edges.

Details of Harmonographs.

The reader may be interested in details of the apparatus shown in Figs. 168 and 170, made by the writer.

The Rectilinear Harmonograph, shown in Fig. 168, has pendulums of 5/8-inch wood, 40 inches long, suspended 30 inches from the lower ends, and set 10 inches apart, centre to centre. The suspensions are of the point type. The weights scale 5 lbs. each. The platform pendulum is provided with a second weight, which can be affixed above the suspension to slow that pendulum for 2:3, 4:5, 7:8, and higher harmonies.

The baseboard is plain, and when the apparatus is in action its ends are supported on boxes or books laid on two tables, or on other convenient supports. The whole apparatus can be taken to pieces very quickly for transport. The total cost of materials used did not exceed 3s. 6d.

The Twin Elliptic Pendulum of Fig. 170 is supported on a tripod base made of three pieces of 1-1/2 x 1-1/2 inch wood, 40 inches long, with ends cut off to an angle of 72 degrees to give a convenient straddle, screwed at the top to an oak head 3/4 inch thick, and braced a foot below the top by horizontal crossbars 2 inches wide and 1/2 inch thick. For transport this stand can be replaced by a flat baseboard similar to that of the Rectilinear Harmonograph described in the last paragraph.

The main pendulum is a straight ash rod, 33 inches long and 1-1/4 inches in diameter, suspended 13-1/2 inches from its upper end. Two weights of 4-1/2 lbs. each, made of rolled sheet lead, are provided for this pendulum. According to the nature of the harmony, one only, or both together below the suspension, or one above and one below, are used.

The weight of the lower pendulum, or deflector, is supported on a disc, resting on a pin passing through the bottom of a piece of brass tubing, which is provided with an eye at its upper end. This eye is connected by a hook with several strands of silk thread, which are attached to the upper pendulum by part of a cycle tyre valve. The stem part of the valve was cut off from the nut, and driven into a suitably sized hole in the end of the main pendulum. The screw collar for holding the valve in place had a little brass disc soldered to the outside, and this disc was bored centrally for the threads to pass through. The edges of the hole had been rounded off carefully to prevent fraying of the threads. (Fig. 177.) The over-all length of the pendulum, reckoning from the point of suspension, is 20 inches. The weights of the lower pendulum are several in number, ranging from l lb. to 3 lbs.



Working the Harmonograph.—A preliminary remark is needed here. Harmonies are, as we have seen, a question of ratio of swing periods. The larger the number of swings made by the more quickly moving pendulum relatively to that of the slower pendulum in a given time, the higher or sharper is the harmony said to be. Thus, 1:3 is a higher harmony than 1:2, and 2:3 is lower or flatter than 3:8.

The tuning of a harmonograph with independent pendulums is a simple matter. It is merely necessary to move weights up or down until the respective numbers of swings per minute bear to one another the ratio required. This type of harmonograph, if made of convenient size, has its limitations, as it is difficult to get as high a harmonic as 1:2, or the octave with it, owing to the fact that one pendulum must in this case be very much shorter than the other, and therefore is very sensitive to the effects of friction.



The action of the Twin Elliptic Pendulum is more complicated than that of the Rectilinear, as the harmony ratio is not between the swings of deflector and upper pendulum, but rather between the swings of the deflector and that of the system as a whole. Consequently "tuning" is a matter, not of timing, but of experiment.

Assuming that the length of the deflector is kept constant—and in practice this is found to be convenient—the ratios can be altered by altering the weights of one or both pendulums and by adjustment of the upper weight.

For the upper harmonies, 1:4 down to 3:8, the two pendulums may be almost equally weighted, the top one somewhat more heavily than the other. The upper weight is brought down the rod as the ratio is lowered.

To continue the harmonies beyond, say, 2:5, it is necessary to load the upper pendulum more heavily, and to lighten the lower one so that the proportionate weights are 5 or 6:1. Starting again with the upper weight high on the rod, several more harmonies may be established, perhaps down to 4:7. Then a third alteration of the weights is needed, the lower being reduced to about one-twentieth of the upper, and the upper weight is once more gradually brought down the rod.

Exact figures are not given, as much depends on the proportions of the apparatus, and the experimenter must find out for himself the exact position of the main weight which gives any desired harmonic. A few general remarks on the action and working of the Twin Elliptic will, however, be useful.

1. Every ratio has two forms.

(a) If the pendulums are working against each other— antagonistically—there will be loops or points on the outside of the figure equal in number to the sum of the figures in the ratio.

(b) If the pendulums are working with each other—concurrently—the loops form inside the figure, and are equal in number to the difference between the figures of the ratio. To take the 1:3 ratio as an example. If the tracing has 3+1=4 loops on the outside, it is a specimen of antagonistic rotation. If, on the other hand, there are 3-1=2 loops on the inside, it is a case of concurrent rotation. (Fig. 176, A.)

2. Figures with a ratio of which the sum of the numbers composing it is an even number (examples, 1:3, 3:5, 3:7) are symmetrical, one half of the figure reproducing the other. If the sum is Uneven, as in 1:2, 2:3, 2:7, the figure is unsymmetrical. (Fig. 177, A.)

3. The ratio 1:3 is the easiest to begin upon, so the experimenter's first efforts may be directed to it. He should watch the growth of the figure closely, and note whether the repeat line is made in front of or behind the previous line of the same loop. In the first case the figure is too flat, and the weight of the upper pendulum must be raised; in the second case the weight must be lowered. Immediately an exact harmonic is found, the position of the weight should be recorded.

Interesting effects are obtained by removing the lower pendulum and allowing the apparatus to describe two elliptical figures successively, one on the top of the other, on the same card. The crossing of the lines gives a "watered silk" appearance to the design, which, if the pen is a very fine one and the lines very close together, is in many cases very beautiful.

Readers who wish for further information on this fascinating subject are recommended to purchase "Harmonic Vibrations," published by Messrs. Newton and Co., 72 Wigmore Street, London, W. This book, to which I am much indebted, contains, besides much practical instruction, a number of charming reproductions of harmonograms.

Before closing this chapter I should like to acknowledge the kind assistance given me by Mr. C. E. Benham, who has made a long and careful study of the harmonograph.



XXXII. A SELF-SUPPLYING MATCHBOX.

This useful little article can be constructed in a couple of hours by a handy person. In general idea it consists of a diamond-shaped box to hold vestas, working up and down diagonally on a vertical member (A in Fig. 179 (1)), which passes through slits at the top and bottom, and runs in grooves cut in the sides of the box. The top of A is grooved to allow a match to rest on it. When the box is drawn up to the full extent allowed by a transverse pin in the slot shown in Fig. 179 (2), the groove is at the lowest point of the box, and is covered by the matches. When the box is lowered, A catches a vesta and takes it up through the top, as seen in Fig. 178, for removal by the fingers.

The only materials required are a cigar-box, some pins, and a supply of glue. The box should be carefully taken to pieces, and the parts soaked in hot water till freed of all paper, and then allowed to dry under pressure, small slips of wood being interposed across the grain to keep them separate and permit the passage of air.



When the wood is dry, cut out with a fret saw two pieces shaped like Fig. 179 (3), to form the ends of the box. Allow a little surplus, so that the edges may be finished off neatly with chisel and plane. The two ends should match exactly, or there will be trouble at a later stage.

Now cut, down the centre of each a groove for one edge of A to run in. By preference it should be square; but if you do not possess the necessary chisel, a V groove made with a knife will suffice—and, of course, in this case the edges of A will have to be bevelled to fit.



The four sides of the box, BB and CC, are next cut out. Their sectional shape is shown in Fig. 179 (1). They should be rather longer than the length of the ordinary vesta, and all of exactly the same length, and rectangular. A very small hack saw (costing about 1s.) with fine teeth is the best possible tool for close cutting, and a small 1 shilling iron plane is invaluable for truing and bevelling the edges.

The glue pot, which we will assume to be ready for use, is now needed to attach the fixed B (the other B is hinged to form a lid for filling the box through) and CC to the ends. This operation must be carried out accurately, so that the slots may not be blocked.

While the glue is setting, cut out A, allowing an extra 1/16 inch of width for fitting. The slot down the centre is best made with a fret saw, and should be smoothed internally by drawing a strip of fine glass paper to and fro through it. The length of the slot is of great importance. It must reach to just that distance from the top edge which brings that edge flush with the bottom of the box when the box is raised; and in the other direction must permit the box to settle on to its foot, so that the match lifted shall project above the box.

Work the edges of A down carefully (double-bevelling them if the notches are V-shaped) till A will run easily, but not loosely, in the box. Then cut out two slips, DD, and bevel them at the top to an angle of 45 degrees. Put A in place and glue them on, taking care that the glue does not hold them fast to A.

Pierce a small hole through DD, in line with the slot, and insert a pin. Draw the box fully up, and see if the top of A sinks to the proper place. If it projects a little, lengthen the slot a trifle.

Cut out the supports EE, finish them neatly, and glue them to A. Make sure that the pin lets the box touch them.

Fix on the lid B with two pins for pivots, and fit a little catch made of brass wire. To give extra security, drive ordinary pins, cut off to 5/8 inch, through the sides into fixed B, CC, and DD, and through EE into A. This is an easy enough business if pilot holes are made with a very fine awl or a tiny drill, and a small, light hammer is used. It now remains only to go over the whole box with glass paper or emery cloth, and to glue a diamond of coarse glass paper to one end for striking the matches on.

Note that the lid must not be opened when the box is down, as it would be wrenched off its pivots.



XXXIII. A WOODEN WORKBOX.

The box illustrated by Fig. 181 was copied from an article of Norwegian manufacture. Its construction is an extremely simple matter, provided that one can get a piece of easily bent wood (birch, for instance), not exceeding 3/16 inch in thickness, for the sides.



The bottom of the box is made of 5/16 or 3/8 inch wood, cut to an oval or elliptical shape. To mark out an ellipse about 8 inches long and 5-1/2 inches wide—this will be a. convenient size—stick two pins into the board 5-1/8 inches apart, pass a loop of thread 14 inches in circumference round these, and run the point of a pencil round the pins in the path which it has to take when confined by the slack of the loop (Fig. 180). Fret-saw along the line.

The wood strip for the side is 4-1/2 inches deep, and 1-1/2 inches longer than the circumference of the bottom. The ends are thinned off somewhat, as shown in Fig. 181, to prevent the lap having a clumsy appearance, and the surface is smoothed all over with sandpaper. Bore a number of small nail holes 3/16 inch from one edge, and then steam the wood over a big saucepan or other suitable vessel until it is quite lissom.

When attaching the side piece to the bottom, begin at the middle, and work first towards what will be the inside end of the lap, and then towards the outside end. Nails are driven in through the holes already drilled. When nailing is finished, clip the top of the overlap with a hand-vice or screw spanner, to prevent the tops of the ends sliding over one another, and bore a line of holes l/4 inch apart, and at the same distance from the outer end. Fine copper wire drawn to and fro through alternate holes from one end of the row to the other and back again, will secure the joint.

The lid overlaps the side 1/4 inch in all directions and has a square notch cut in it at one end to pass under the piece A, and at the other a deeper, circular-ended nick to enable it to pass over the key B when that is turned into the position shown in the illustration. A is cut out of 1/4-inch wood; B, in one piece, out of 1/2-inch. Their length under the heads exceeds the inside depth of the box by the thickness of the lid.

A is affixed rigidly to the side by small screws or wire, while B must be attached in a manner, which will allow the head to rotate. Cut two nicks round the shank, and two horizontal slots at the same height through the end of the box. A couple of brass rings must then be procured of such a size that, when flattened into a somewhat oval shape, they will project beyond the slots sufficiently to allow a piece of wire to pass through them and prevent their being drawn back again.

Quarter-inch wood will do for the lid. A handle is made out of a couple of inches of small cane bent into a semicircle, let through the lid at each end, glued, and cut off flush.

The exterior may be decorated by a design in poker-work, or be stained and varnished. This is left to the maker's discretion.



XXXIV. WRESTLING PUPPETS.



The expenditure of a halfpenny, and a quarter of an hour's use of a pocket knife, bradawl, and pliers, will produce a toy which is warranted to amuse grown-ups as well as children. Wrestlers made out of clothes pegs may be bought for a copper or two in the street, and are hardly a novelty; yet a few notes on home production will not be a waste of space, as making is cheaper, and much more interesting, than buying.

The clothes pegs used must be of the shape shown in Fig. 182, with a round top. They cost one penny per dozen.

Drill holes through body and legs as indicated in Fig. 182. Cut the legs from the "trunk,'" and whittle them to the shape of Fig. 183. The arms, made out of any thin wood, are 2-1/4 inches long between centres of end holes.

To get the best results the two arms and the four legs should be paired off to exactly the same length.



The neatest method of attaching the parts is to use small brass tacks, which must, of course, be of somewhat larger diameter than the holes in the body. Holes in arms and legs are a loose fit, so that the wrestlers may be very loose-jointed, and the tacks must not be driven in far enough to cause any friction.

Instead of tacks one may use wire passed through the parts and secured by a bend or loop at each end. Wire has the disadvantage of entangling the thread which works the figures.

When assembling is finished, bore holes in the centres of the arm pieces, pass a piece of wire through, and twist it into a neat loop at each end. To one loop tie 2 feet of strong thread (carpet thread is best), and to the free end of the thread a large nail or hook. The other loop has 6 feet or so of thread tied to it, to be worked by the hand. If the thread is stained black, it will be practically invisible by artificial light.

The nail or hook is stuck under the edge of the carpet, or into some crack or cranny which affords a good hold, and the wrestlers are worked by motions of the hand. The funniest antics are produced by very slight jerks.

If the arms are set too close together the heads may stick between them, in which case one must either flatten off the sides of the heads or insert fresh arm wires of greater length. If a head persists in jamming against the thread wire or getting under it and staying there, cut 1/2 inch off a pin and stick it into the front of the crown, so that the head is arrested by the wire when the wrestler bends forward.



Large Wrestlers.—A more elaborate and realistic pair is shown in Fig. 184. The originals of the sketch are 8 inches high. Half-inch deal was used for the bodies, 3/8-inch for the legs and arms. The painting-in of hair, features, tights, and shoes adds considerably to the effect. The heads and limbs are mere profiles, but anyone with a turn for carving might spend a little time in rounding off and adding details which will make the puppets appear more lifelike.



XXXV. DOUBLE BELLOWS.

The small-sized bellows which have become popular in sitting-rooms are usually more ornamental than efficient, and make one think regretfully of the old-fashioned article of ample capacity which is seldom seen nowadays.

Fig. 185 illustrates a method of coupling up two small bellows in such a manner as to provide an almost continuous blast, besides doubling the amount of air sent through the fire in a given time, at the coat of but little extra exertion. A piece of wood half an inch thick is screwed across one bellows just behind the valve hole. The two bellows are then laid valve facing valve, and are attached to one another by a strip of tin passed round the wood just behind the nozzles and by tying the two fixed handles together.



Make a rectangle of stout wire somewhat wider than the handles and long enough to reach from the outer face of one moving handle to that of the other, when one bellows is quite closed and the other full open. The ends of the wire should be soldered together, and the ends of the link held up to the handles by a couple of staples.

An alternative method is to use a piece of wood with a screw driven into it at right angles near each end through the staples on the handles (Fig. 185, a). In place of the staples you may use screw-in eyes fitting the screws.



XXXVI. A HOME-MADE PANTOGRAPH.

The pantograph is a simple apparatus for copying drawings, maps, designs, etc., on a reduced or enlarged scale, or to the same size as the original.



A sketch of a pantograph is given in Fig. 186. Four rods are jointed together to form a parallelogram, the sides of which can be lengthened or shortened to suit the scale of reproduction. One is attached by a fixed pivot at a to the board on which the drawing is done. At b and e are removable pivots, used for adjusting the rods; at c is a pivot which projects an inch or so below the rods. The pointer is inserted at d for enlargement, or at f for reduction, the pencil being in the unoccupied hole at d or f.

If a same-sized copy is desired, the fixed pivot is transferred to d, and the pencil and pointer placed at a and f respectively.

Construction of an Enlarging and Reducing Pantograph.—Cut out of 1/8-inch oak, walnut, or beech four rods 5/8 inch wide and 19 inches long. Smooth them well all over, and make marks near the ends of each, exactly 18 inches apart. The graduation of the rods for the adjustment pivot holes is carried out in accordance with the measurements given in Fig. 187. It is advisable to mark out and bore each rod separately if you do not possess a machine which will drill holes quite perpendicularly; if you do, all four rods can be drilled at one operation.

In Fig. 187 the lower row of numerals indicates the number of times (in diameters) the original is enlarged when all four holes similarly figured are used; the upper row, the size of the copy as compared with the original in case of reduction.

If proportions other than those given are required, a very little calculation will locate the necessary holes.

Pivots.—All the pivots must fit their holes accurately, as any looseness at the joints detracts from the truth of reproduction. For pivots band b and e may use brass screws and small pieces of hard wood as nuts to hold them in position. The nuts should screw on rather stiffly, and not be forced hard against the rods, as free motion with little friction at all joints is essential for good work.



The fixed pivot at a may be merely the shank of a wire nail of the proper size driven into the board, a cork collar being slipped over it to keep the rod the proper distance from the board. For c use a screw to the head of which has been soldered half an inch of a round-headed brass nail, which will move easily over the paper. At d is needed a hollow pivot, fashioned out of a quarter of an inch of pencil-point protector or some other thin tube, burred over slightly at the ends so as not to fall out. The end of B at f has a slotted hole to grip the pencil or pointer, as the case may be.