Things To Make

by Archibald Williams

CONSTRUCTION.

After these general remarks, we may proceed to a practical description of manufacture, which will apply to kites of all dimensions. It will be prudent to begin on small models, as requiring small outlay.

Having decided on the size of your kite, cut out two pieces of material as wide as a box is to be deep, and as long as the circumference of the box plus an inch and a half to spare. Machine stitch 5/8 inch tapes along each edge, using two rows of stitching about 1/8 inch from the edges of the tape. Then double the piece over, tapes inside, and machine stitch the ends together, three quarters of an inch from the edge. Note.—All thread ends should be tied together to prevent unravelling, and ends of stitching should be hand-sewn through the tape, as the greatest strain falls on these points.

The most convenient shape for the rods is square, as fitting the corners and taking tacks most easily. The sectional size of the rods is governed by the dimensions of the kite, and to a certain extent by the number of stretchers used. If four stretchers are employed in each box, two near the top and two near the bottom, the rods need not be so stout as in a case where only a single pair of central stretchers is preferred.

Lay the two boxes flat on the floor, in line with one another, and the joins at the same end. Pass two rods through, and arrange the boxes so that the outer edges are 1/2 inch from the ends of the rods. (These projections protect the fabric when the kite strikes the ground).

Lay the rods on one corner, so that the sides make an angle of 45 degrees with the floor, pull the boxes taut—be careful that they are square to the rods—and drive three or four tacks through each end of the box into the rods. Then turn them over and tack the other sides similarly. Repeat the process with the other rods after measuring to get the distances correct.

The length of the stretchers is found approximately by a simple arithmetical sum, being the square root of the sum of the squares of the lengths of two adjacent sides of the box. For example, if each box is 20 by 15 inches, the diagonal is the square root of (20 squared plus 15 squared) = square root of 625 = 25 inches. The space occupied by the vertical rods will about offset the stretch of the material, but to be on the safe side and to allow for the notches, add another half-inch for small kites and more proportionately for large ones. It is advisable to test one pair of stretchers before cutting another, to reduce the effect of miscalculations.

The stretcher notches should be deep enough to grip the rods well and prevent them twisting, and one must take care to have those on the same stretcher exactly in line, otherwise one or other cannot possibly “bed” properly. A square file is useful for shaping the notches.

Ordinarily stretchers do not tend to fall out, as the wind pressure puts extra strain on them and keeps them up tight. But to prevent definitely any movement one may insert screw eyes into the rods near the points at which the stretchers press on them, and other eyes near the ends of the stretchers to take string fastenings. These attachments will be found useful for getting the first pair of stretchers into position, and for preventing the stretchers getting lost when the kite is rolled up.

The bridle is attached to four eyes screwed into the rods near the tops of the boxes. (See Fig. 118.) The top and bottom elements of the bridle must be paired off to the correct length; the top being considerably shorter than the bottom. All four parts may be attached to a brass ring, and all should be taut when the ring is pulled on. The exact adjustment must be found by experiment. In a very high wind it is advisable to shorten the top of the bridle if you have any doubt as to the strength of your string, to flatten the angle made by the kite with the wind.

[Illustration: FIG. 115.—Details of stretcher attachment for diamond-shaped box kites.]

Diamond Box Kites.—In another type of box kite the boxes have four equal sides, but the boxes are rhombus-shaped, as in Fig. 116, the long diagonal being square to the wind, and the bridle attached at the front corner.

For particulars of design and construction I am much indebted to Mr. W. H. Dines, F.R.S., who has used the diamond box kite for his meteorological experiments to carry registering meteorographs several thousands of feet into the air.

The longitudinal sticks used at the corners have the section shown in Fig. 115. They are about four times as wide at the front edge, which presses against the fabric, as at the back, and their depth is about twice the greater width. This shape makes it easy to attach the shorter stretchers, which have their ends notched and bound to prevent splitting.

[Illustration: FIG. 116.—Plan of diamond box kite, showing arrangement of stretchers.]

Fig. 117 is a perspective diagram of a kite. The sail of each box measures from top to bottom one-sixth the total circumference of the box, or, to express the matter differently, each face of the box is half as long again as its depth. The distance separating the boxes is equal to the depth of a box.

The sides of a box make angles of 60 degrees and 120 degrees with one another, the depth of the space enclosed from front to back being the same as the length of a side. With these angles the effective area of the sails is about six-sevenths of the total area. Therefore a kite of the dimensions given in Fig. 117 will have an effective area of some thirty square feet.

[Illustration: FIG. 117.—Diamond box kite in perspective. Ties are indicated by fine dotted lines.]

The long stretchers pass through holes in the fabric close to the sticks, and are connected with the sticks by stout twine. Between stretcher and stick is interposed a wedge-shaped piece of wood (A in Fig. 115), which prevents the stick being drawn out of line. This method of attachment enables the boxes to be kept tight should the fabric stretch at all—as generally happens after some use; also it does away with the necessity for calculating the length of the stretchers exactly.

The stretchers are tied together at the crossing points to give support to the longer of the pair.

The dotted lines AB, AC, AD, EM, and EN in Fig. 117 indicate ties made with wire or doubled and hemmed strips of the fabric used for the wings. AB, running from the top of the front stick to the bottom of the back stick, should be of such a length that, when the kite is stood on a level surface, the front and back sticks make right angles with that surface, being two sides of a rectangle whereof the other two sides are imaginary lines joining the tops and bottoms of the sticks. This tie prevents the back of the kite drooping under pressure of the wind, and increases the angle of flight. The other four ties prevent the back sails turning over at the edges and spilling the wind, and also keep them flatter. This method of support should be applied to the type of kite described in the first section of this chapter.

String Attachment.—A box kite will fly very well if the string is attached to the top box only. The tail box is then free to tilt up and trim the kite to varying pressures independently of the ascent of the kite as a whole. When the bottom box also is connected to the string it is a somewhat risky business sending a kite up in a high wind, as in the earlier part of the ascent the kite is held by the double bridle fairly square to the wind. If any doubt is entertained as to the ability of the string to stand the pressure, the one-box attachment is preferable, though possibly it does not send the kite to as great a height as might be attained under similar conditions by the two-box bridle.

[Illustration: FIG. 118.—Box kite with rear wings.]

When one has to attach a string or wire to a large kite at a single point, the ordinary method of using an eye screwed into the front stick is attended by obvious risks. Mr. Dines employs for his kites (which measure up to nine feet in height) an attachment which is independent of the front stick. Two sticks, equal in length to the width of the sail, are tacked on to the inner side of the sail close to the front stick. Rings are secured to the middle of the sticks and connected by a loop of cord, to which the wire (in this case) used for flying the kite is made fast.

A Box Kite with Wings.—The type of kite shown in Fig. 118 is an excellent flyer, very easy, to make and very portable. The two boxes give good longitudinal stability, the sides of the boxes prevent quick lateral movements, and the two wings projecting backwards from the rear corners afford the “dihedral angle” effect which tends to keep the kite steadily facing the wind. The “lift,” or vertical upward pull, obtained with the type is high, and this, combined with its steadiness, makes the kite useful for aerial photography, and, on a much larger scale, for man-lifting.

The materials required for the comparatively small example with which the reader may content himself in the first instance are:

8 wooden rods or bamboos, 4 feet long and 1/2 inch in diameter. 4 yards of lawn or other light, strong material, 30 inches wide. 12 yards of unbleached tape, 5/8 inch wide. 8 brass rings, 1 inch diameter.

The Boxes.—Cut off 2 yards 8 inches of material quite squarely, fold down the middle, crease, and cut along the crease. This gives two pieces 80 by 15 inches.

Double-stitch tape along the edges of each piece.

Lay the ends of a piece together, tapes inside, and stitch them together half an inch from the edge. Bring a rod up against the stitching on the inside, and calculate where to run a second row of stitching parallel to the first, to form a pocket into which the rod will slip easily but not loosely. (See Fig. 119, a.)

Remove the rod and stitch the row.

Now repeat the process at the other end of the folded piece. The positions of the other two rod pockets must be found by measuring off 15 inches from the inner stitching of those already made. (Be careful to measure in the right direction in each case, so that the short and long sides of the box shall be opposite.) Fold the material beyond the 15-inch lines to allow for the pockets and the 1/2-inch “spare,” and make the two rows of stitching.

[Illustration: FIG. 119.—Plan of box kite with rear wings.]

Repeat these operations with the second strip of material, and you will have prepared your two boxes, each measuring, inside the pockets, 15 by about 20 inches. (See Fig. 119.) Now cut out the wings in accordance with the dimensions given in Fig. 120. Each is 47-1/2 inches long and 15 inches across at the broadest point. It is advisable to cut a pattern out of brown paper, and to mark off the material from this, so arranging the pattern that the long 47-1/2-inch side lies on a selvedge. [The edge of a fabric that is woven so that it will not fray or ravel.]

[Illustration: FIG. 120.—Wing for box kite.]

Double stitch tapes along the three shorter sides of each wing, finishing off the threads carefully. Then sew the wings to what will be the back corners of the boxes when the kite is in the air—to the “spares” outside the rod pockets of a long side.

Take your needle and some strong thread, and make all corners at the ends of pockets quite secure. This will prevent troublesome splitting when the kite is pulling hard.

Sew a brass ring to each of the four wing angles, AA, BB, at the back, and as many on the front of the spares of the rod pockets diagonally opposite to those to which the wings are attached, halfway up the boxes. These rings are to take the two stretchers in each box.

Slip four rods, after rounding off their ends slightly, through the pockets of both boxes, and secure them by sewing the ends of the pockets and by the insertion of a few small tacks. These rods will not need to be removed.

The cutting and arrangement of the stretchers and the holes for the same require some thought. Each stretcher lies behind its wing, passes in front of the rod nearest to it, and behind that at the corner diagonally opposite. (See Fig. 119.) The slits through which it is thrust should be strengthened with patches to prevent ripping of the material.

Two persons should hold a box out as squarely as possible while a stretcher is measured. Cut a nick 3/8 inch deep in one end of the stretcher, and pass the end through the fabric slits to the ring not on the wing. Pull the wing out, holding it by its ring, and cut the stretcher off 1 inch from the nearest point of the ring. The extra length will allow for the second nick and the tensioning of the material. Now measure off the second stretcher by the first, nick it, and place it in position. If the tension seems excessive, shorten the rods slightly, but do not forget that the fabric will stretch somewhat in use.

[Illustration: FIG. 121.—Box kite with front and back wings.]

Make the stretchers for the second box, and place them in position. The wings ought to be pretty taut if the adjustments are correct, but should they show a tendency to looseness, a third pair of stretchers of light bamboo may be inserted between the other two, being held up to the rods by loops of tape. In order to be able to take up any slackness, the wing end of each stretcher may be allowed to project a couple of inches, and be attached by string to the near ring, as described on p. 271. The bridle to which the flying string is attached is made up of four parts, two long, two short, paired exactly as regards length. These are attached to eyes screwed into the front rods three inches below the tops of the boxes. Adjustment is made very easy if a small slider is used at the kite end of each part. These sliders should be of bone or some tough wood, and measure 1 inch by 3/8 inch. The forward ends of the bridle are attached to a brass ring from which runs the flying string.

It is advisable to bind the stretchers with strong thread just behind the notches to prevent splitting, and to loosen the stretchers when the kite is not in use, to allow the fabric to retain as much as possible of its elasticity.

The area of the kite affected by wind is about 14 square feet; the total weight, 1-1/2 lb. The cost of material is about 2s.

The experience gained from making the kite described may be used in the construction of a larger kite, six or more feet high, with boxes 30 by 22 by 22 inches, and wings 24 inches wide at the broadest point. If a big lift is required, or it is desired to have a kite usable in very light breezes, a second pair of wings slightly narrower than those at the back may be attached permanently to the front of the boxes, or be fitted with hooks and eyes for use on occasion only. (Fig. 121.) In the second case two sets of stretchers will be needed.

[Illustration: FIG. 122.—Simple string winder for kite.]

Note.—If all free edges of boxes and wings are cut on the curve, they will be less likely to turn over and flap in the wind; but as the curvature gives extra trouble in cutting out and stitching, the illustrations have been drawn to represent a straight-edged kite.

Kite Winders.—The plain stick which small children flying small kites on short strings find sufficient for winding their twine on is far too primitive a contrivance for dealing with some hundreds of yards, may be, of string. In such circumstances one needs a quick-winding apparatus. A very fairly effective form of winder, suitable for small pulls, is illustrated in Fig. 122.

Select a sound piece of wood, 3/8-inch thick, 5 inches wide, and about 1 foot long. In each end cut a deep V, the sides of which must be carefully smoothed and rounded with chisel and sandpaper. Nail a wooden rod, 15 inches long and slightly flattened where it makes contact, across the centre of the board, taking care not to split the rod, and clinch the ends of the nails securely. The projecting ends of the rods are held in the hands while the string runs out. The projecting piece, A, which must also be well secured, is for winding in. The winding hand must be held somewhat obliquely to the board to clear the spindle. Winding is much less irksome if a piece of tubing is interposed between the spindle and the other hand, which can then maintain a firm grip without exercising a braking effect.

This kind of winder is unsuited for reeling in a string on which there is a heavy pull, as the hands are working at a great disadvantage at certain points of a revolution.

[Illustration: FIG. 123.—Plan of string-winding drum, frame, and brake.]

A far better type is shown in Figs. 123 and 124. Select a canister at least 6 inches in diameter, and not more than 6 inches long, with an overlapping lid. Get a turner to make for you a couple of wooden discs, 3/8 inch thick, and having a diameter 2 inches greater than that of the tin. Holes at least 3/8 inch across should be bored in the centre of each. Cut holes 1 inch across in the centre of the lid and the bottom of the canister, and nail the lid concentrically to one disc, the canister itself to the other. Then push the lid on the tin and solder them together. This gives you a large reel. For the spindle you will require a piece of brass tubing or steel bar 1 foot long and large enough to make a hard driving fit with the holes in the wood. Before driving it in, make a framework of 3/4-inch strip iron (Fig. 123), 3/32 or 1/8 inch thick, for the reel to turn in. The width of this framework is 1 inch greater than the length of the reel; its length is twice the diameter of the canister. Rivet or solder the ends together. Halfway along the sides bore holes to fit the spindle.

Make a mark 1 inch from one end of the spindle, a second l/8 inch farther away from the first than the length of the reel. Drill 3/16-inch holes at the marks. Select two wire nails which fit the holes, and remove their heads. Next cut two 1/4-inch pieces off a tube which fits the spindle. The reel, spindle, and framework are now assembled as follows:

[Illustration: FIG. 124.—End view of string winder, showing brake and lever.]

Push the end of the spindle which has a hole nearest to it through one of the framework holes, slip on one of the pieces of tubing, drive the spindle through the reel until half an inch projects; put on the second piece of tubing, and continue driving the spindle till the hole bored in it shows. Then push the nails half-way through the holes in the spindle, and fix them to the ends of the reel by small staples. A crank is made out of 1/2-inch wood (oak by preference) bored to fit the spindle, to which it must be pinned. A small wooden handle is attached at a suitable distance away. If there is any fear of the wood splitting near the spindle, it should be bound with fine wire. An alternative method is to file the end of the spindle square, and to solder to it a piece of iron strip in which a square hole has been made to fit the spindle. The crank should be as light as is consistent with sufficient strength, and be balanced so that there shall not be unpleasant vibration when the string runs out fast, and of course it must be attached very securely to the spindle.

What will be the front of the framework must be rounded off on the top edge, which has a wire guide running parallel to it (Fig. 123) to direct the string on to the reel; and into the back are riveted a couple of eyes, to which are attached the ends of a cord passing round the body, or some stationary object.

[Illustration: FIG. 125.—String winder in operation.]

A pin should be provided to push into a hole at one end of the reel and lock the reel by striking the framework, and it will be found a great convenience to have a brake for controlling the reel when the kite is rising. Such a brake is easily fitted to the side of the frame, to act on the left end of the reel when a lever is depressed by the fingers. There should be a spring to keep it off the reel when it is not required. The diagrams show where the brake and brake lever are situated.

Note.—To obtain great elevations a fine wire (piano wire 1/32 inch in diameter) is generally used, but to protect the user against electric shocks the wire must be connected with an “earthed” terminal, on the principle of the lightning conductor.

XXIV.
PAPER GLIDERS.

In this chapter are brought to your notice some patterns of paper gliders which, if made and handled carefully, prove very satisfactory. Gliders are sensitive and “moody” things, so that first experiments may be attended by failure; but a little persistence will bring its reward, and at the end of a few hours you will, unless very unlucky, be the possessor of a good specimen or two.

The three distinguishing features of a good glider are stability, straightness of flight, and a small gliding angle. If the last is as low as 1 in 10, so that the model falls but 1 foot vertically while progressing 10 feet horizontally, the glider is one to be proud of.

Materials.—The materials needed for the gliders to be described are moderately stout paper—cream-laid notepaper is somewhat heavy for the purpose—and a little sealing wax or thin sheet metal for weighting.

[Illustration: FIG. 126.—Paper glider: Model “A.”]

[Illustration: FIG. 127.—How to launch Model “A.”]

Model “A.”—Double a piece of paper 8 inches long and 2-1/2 inches wide, and cut out, through both folds, the shape shown in Fig. 126. Flatten the piece and fold the “head” inwards four times on the side away from the direction in which the paper was folded before being cut out. Flatten the folds and fix to the centre a little clip formed by doubling a piece of thin metal 3/16 by 1/2 inch. Make certain that the wings are quite flat, and then, holding the glider between thumb and first finger, as shown in Fig. 127, push it off gently. If the balance is right, it will fly quite a long way with an undulating motion. If too heavy in front, it will dive; if too light, it will rise suddenly and slip backwards to the ground. The clip or the amount of paper in the head must be modified accordingly. This type is extraordinarily efficient if the dimensions, weighting, and shape are correct, and one of the easiest possible to make.

Model “B.”—The next model (Fig. 128), suggesting by its shape the Langley steam-driven aeroplane, has two sets of wings tandem. Double a piece of paper and cut out of both folds simultaneously a figure of the shape indicated by the solid lines in the diagram. The portion A is square, and forms the head weight; B indicates the front planes, C the rear planes. Bend the upper fold of each pair into the positions B1, C1, marked by dotted lines. Their front edges make less than a right angle with the keel, to ensure the wings slanting slightly upwards towards the front when expanded.

The model is now turned over, and the other wings are folded exactly on top of their respective fellows. Then the halves of the head are folded twice inwards, to bring the paper into as compact a form as possible. It remains to open out the wings at right angles to the keel, and then raise their tips slightly so that the two planes of a pair shall make what is called a “dihedral” angle with one another.

[Illustration: FIG. 128.—Details of paper gliders: Model “B” above,
Model “C” below.]

Before launching, look at your model endways and make sure that the rear planes are exactly in line with those in front. It is essential that they should be so for straight flight. Then grip the keel at its centre between finger and thumb and launch gently. Mark how your glider behaves. If it plunges persistently, trim off a very little of the head. If, on the contrary, it settles almost vertically, weight must be added in front. The position of the weight is soon found by sliding a metal clip along the keel until a good result is obtained.

Note that if the leading edges of the front wings are bent slightly downwards the glider may fly much better than before.

A good specimen of this type is so stable that if launched upside down it will right itself immediately and make a normal flight.

Model “C.”—This is cut out of doubled paper according to the solid lines of Fig. 128. The three sets of planes are bent back in the manner already described, but the front planes are given a somewhat steeper angle than the others. This type is very stable and very fairly efficient.

General Remarks.—Always pick up a glider by the keel or middle, not by one of the wings, as a very little distortion will give trouble.

The merits of a glider depend on length, and on straightness of flight; so in competition the launching height should be limited by a string stretched across the room, say 6 feet above the floor. If the room be too short for a glider to finish its flight, the elevation at which it strikes the wall is the measure of its efficiency.

Out-of-door flights are impracticable with these very frail models when there is the slightest breeze blowing. On a perfectly calm day, however, much better fun can be got out of doors than in, owing to the greater space available. A good glider launched from a second-floor window facing a large lawn should travel many yards before coming to grass.

Large gliders of the types detailed above can be made of very stout paper stiffened with slips of cane or bamboo; but the time they demand in construction might perhaps be more profitably spent on a power-driven aeroplane such as forms the subject of the next chapter.

XXV.
A SELF-LAUNCHING MODEL AEROPLANE.

By V. E. Johnson, M.A.

This article deals not with a scale model—a small copy of some full-sized machine—but with one designed for actual flight; with one not specially intended to create records either of length or duration, but which, although small details must perforce be omitted, does along its main lines approximate to the “real thing.”

Partly for this reason, and partly because it proves a far more interesting machine, we choose a model able to rise from the ground under its own power and make a good flight after rising, assuming the instructions which we give to have been carefully carried out. It is perhaps hardly necessary to add that such a machine can always be launched by hand when desired.

Before entering into special details we may note some broad principles which must be taken into account if success is to attend our efforts.

Important Points.—It is absolutely essential that the weight be kept down as much as possible. It is quite a mistake to suppose that weight necessarily means strength. On the contrary, it may actually be a cause of weakness if employed in the wrong place and in the wrong way. The heavier the machine, the more serious the damage done in the event of a bad landing. One of the best and easiest ways of ensuring lightness is to let the model be of very simple construction. Such a model is easier to build and more efficient when constructed than one of more complicated design. Weigh every part of your model as you construct it, and do not be content until all symmetrically arranged parts which should weigh the same not only look alike but do actually balance one another. (Note.—The writer always works out the various parts of his models in grammes, not ounces.) If a sufficiently strong propeller bearing weighing only half a gramme can be employed, so much the better, as you have more margin left for some other part of the model in which it would be inadvisable to cut down the weight to a very fine limit.

Details.—To pass now to details, we have four distinct parts to deal with:—

1. The framework, or fuselage.

2. The supporting surfaces, consisting of the main plane, or aerofoil, behind, and the elevator in front.

3. The propellers.

4. The motor, in this case two long skeins of rubber; long, because we wish to be able to give our motor many turns, from 700 to, say, 1,000 as a limit, so that the duration of flight may be considerable.

[Illustration: FIG. 129.-Sections of backbone for model aeroplane.]

The Backbone.—For the backbone or central rod take a piece of pitch pine or satin walnut 52 inches long, 5/8 inch deep, and 1/2 inch broad, and plane it down carefully until it has a T-shaped section, as shown in Fig. 129, and the thickness is not anywhere more than 1/8 inch. It is quite possible to reduce the thickness to even 1/16 inch and still have a sufficient reserve of strength to withstand the pull of 28 strands of 1/16-inch rubber wound up 1,000 times; but such a course is not advisable unless you are a skilful planer and have had some experience in model-making.

If you find the construction of the T-shaped rod too difficult, two courses are open—

(l) To get a carpenter to do the job for you, or

(2) To give the rod the triangular section shown in Fig. 129, each side of the equilateral triangle being half an inch long.

[Illustration: FIG. 150—Side elevation of model aeroplane.]

The top of the T or the base of the triangle, as the case may be, is used uppermost. This rod must be pierced in three places for the vertical masts employed in the bracing of the rod, trussing the main plane, and adjusting the elevator. These are spaced out in Fig. 130, which shows a side elevation of the model. Their sectional dimensions are 1/16 by 1/4 inch; their respective lengths are given in Fig. 130. Round the front edges and sharpen the rear.

In Fig. 130 is shown the correct attitude or standing pose necessary to make the model rise quickly and sweep boldly up into the air without skimming the ground for some 10 to 20 yards as so many models do. E is the elevator (7 by 3 inches); A the main plane (5-1/2 by 29 inches); W the wheels; and RS the rear skid, terminating in a piece of hooked steel wire. The vertical bracing of these masts is indicated. The best material to use for the purpose is Japanese silk gut, which is very light and strong. To brace, drill a small, neat hole in the mast and rod where necessary, pass through, and tie. Do the same with each one.

To return to the central mast, which must also form the chassis. This is double and opened out beneath as shown in Fig. 131, yz being a piece similar to the sides, which completes, the triangle x y z and gives the necessary rigidity. Attach this piece by first binding to its extremities two strips of aluminium, or by preference very thin tinned iron, Tl and T2. Bend to shape and bind to xy, xz as shown in Fig. 131.

[Illustration: FIG. 131.—Front elevation of chassis.]

[Illustration: FIG. l32.-Wheel for model aeroplane chassis.]

[Illustration: FIG. 133.—Plan of model aeroplane.]

The Wheels and Chassis.—WW are the two wheels on which the model runs. They are made of hollow brass curtain rings, 1 inch in diameter, such as can be bought at four a penny. For spokes, solder two strips of thin tinned iron to the rings, using as little solder as possible. (Fig. 132.) To connect these wheels with the chassis, first bind to the lower ends of xy, xz two strips of thin tinned iron, T3 and T4, after drilling in them two holes of sufficient size to allow a piece of steel wire of “bonnet pin” gauge to pass freely, but not loosely, through them. Soften the wire by making it red hot and allowing it to cool slowly, and solder one end of this wire (which must be quite straight and 5-1/4 inches long) to the centre of the cross pieces or spokes of one wheel. Pass the axle through the holes in the ends of xy, xz, and solder on the other wheel. Your chassis is then finished.

The rear skid (RS in Fig. 130) is attached to the central rod by gluing, and drilling a hole through both parts and inserting a wooden peg; or the upright may be mortised in. On no account use nail, tack, or screw. Attach the vertical masts and the horizontal ones about to be described by gluing and binding lightly with thread, or by neatly glued strips of the Hart’s fabric used for the planes.

Horizontal Spars, etc.—To consider now the horizontal section or part plan of the model, from which, to avoid confusion, details of most vertical parts are omitted. Referring to Fig. 133, it will be seen that we have three horizontal masts or spars—HS1, 4 inches; HS2, 6 inches; and HS3, slightly over 12 inches long. The last is well steamed, slightly curved and left to dry while confined in such a manner as to conform to the required shape. It should so remain at least twenty-four hours before being fixed to the model. All the spars are attached by glue and neat cross bindings. If the central rod be of triangular instead of T section, the join can be made more neatly. The same remarks apply to the two 9 and 10 inch struts at the propeller end of the rod, which have to withstand the pull of the rubber motor on PPl. These two pieces will have a maximum strength and minimum weight if of the T section used for the rod. If the work is done carefully, 1/4 inch each way will be sufficient.

Main Plane and Elevator.—The framework of each plane is simply four strips of satin walnut or other suitable wood, 1/4 inch broad and 1/16 inch or even less in thickness for the main plane, and about 1/16 by 1/16 inch for the elevator. These strips are first glued together at the corners and left to set. The fabric (Hart’s fabric or some similar very light material) is then glued on fairly tight—that is, just sufficiently so to get rid of all creases. The main plane is then fixed flat on to the top of the central rod by gluing and cross binding at G and H. (A better but rather more difficult plan is to fasten the rectangular frame on first and then apply the fabric.) The same course is followed in dealing with the elevator, which is fixed, however, not to the rod, but to the 4-inch horizontal spar, HS1, just behind it, in such a manner as to have a slight hinge movement at the back. This operation presents no difficulty, and may be effected in a variety of ways. To set the elevator, use is made of the short vertical mast, M1. A small hole is pierced in the front side of the elevator frame at Z, and through this a piece of thin, soft iron wire is pushed, bent round the spar, and tied. The other end of the wire is taken forward and wrapped three or four times round the mast M1, which should have several notches in its front edge, to assist the setting of the elevator at different angles. Pull the wire tight, so that the elevator shall maintain a constant angle when once set. H H1 is a piece of 25 to 30 gauge wire bent as shown and fastened by binding. It passes round the front of the rod, in which a little notch should be cut, so as to be able to resist the pull of the twin rubber motors, the two skeins of which are stretched between H H1 and the hooks formed on the propeller spindles. If all these hooks are covered with cycle valve tubing the rubber will last much longer. The rubber skeins pass through two little light wire rings fastened to the underside ends of HS2. (Fig. 133.)

The front skid or protector, FS, is made out of a piece of thin, round, jointless cane, some 9 inches in length, bent round as shown in Fig. 134, in which A B represents the front piece of the T-shaped rod and x y z a the cane skid; the portion x y passing on the near side of the vertical part of the T, and z a on the far side of the same. At E and F thread is bound right round the rod. Should the nose of the machine strike the ground, the loop of cane will be driven along the underside of the rod and the shock be minimized. So adjust matters that the skid slides fairly stiff. Note that the whole of the cane is on the under side of the top bar of the T.

[Illustration: FIG. 134.—Front skid and attachment to backbone.]

Bearings.—We have still to deal with the propellers and their bearings. The last, TN and TNl (Fig. 133), are simply two tiny pieces of tin about half a gramme in weight, bent round the propeller spar HS3 at B and B1. Take a strip of thin tin 1/4 inch wide and of sufficient length to go completely round the spar (which is 1/4 by 1/8 inch) and overlap slightly. Solder the ends together, using a minimum amount of solder. Now bore two small holes through wood and tin from rear to front, being careful to go through the centre. The hole must be just large enough to allow the propeller axle to run freely, but not loosely, in it. Primitive though such a bearing may seem, it answers admirably in practice. The wood drills out or is soon worn more than the iron, and the axle runs quite freely. The pull of the motor is thus directed through the thin curved spar at a point where the resistance is greatest—a very important matter in model aeroplane construction. To strengthen this spar further against torsional forces, run gut ties from B and Bl down to the bottom of the rear vertical skid post; and from B to B1 also pass a piece of very thin piano wire, soldered to the tin strips over a little wooden bridge, Q, like a violin bridge, on the top of the central rod, to keep it quite taut.

[Illustration: FIG. 135—“Centrale” wooden propeller.]

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.

[Illustration: FIG. 136.—Axle and hook for propeller.]

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.

[Illustration: FIG. 137.]

[Illustration: FIG. 138.]

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.

[Illustration: FIG. 139.]

[Illustration: FIG. 140.]

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.

[Illustration: FIG. 141.]

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.

[Illustration: FIG. 142.]

[Illustration: FIG. 143.]

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.

[Illustration: FIG. 144.]

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.

[Illustration: FIG. 145.]

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.

[Illustration: FIG. 146.]

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.

[Illustration: FIG. 147.—Apparatus for illustrating an apparent scientific paradox.]

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.

[Illustration: FIG. 148.-Magic pendulums.]

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).

[Illustration: FIG. 149.—Smoke-ring apparatus.]

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.

[Illustration: FIG. 150.—Smoke-making apparatus.]

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.

[Illustration: FIG. 151.—Standard rain-gauge.]

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.

[Illustration: FIG. 152.—Section of homemade rain-gauge.]

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.

[Illustration: FIG. 153.—Self-measuring gauge.]

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.

[Illustration: FIG. 154.—Gauge in case.]

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.

[Illustration: FIG. 165—Wind vane with dial.]

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.

[Illustration: FIG. 156.—Elevation and plan of vane.]

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.

[Illustration: FIG. 157.—Details of bevel gear and arrow.]

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.

[Illustration: FIG. 158.—Plan and elevation of electric contact on vane post.]

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.

[Illustration: FIG. 159.—Magnetic recording 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.

[Illustration: FIG. 160.—Another type of electric dial with compass needle for pointer.]

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.

[Illustration: FIG. 161.—General arrangement of electric wind recorder.]

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.