[Ill.u.s.tration: FIG. 148.-Magic pendulums.]
To make the experiment more attractive, we subst.i.tute 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 subst.i.tute 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 pa.s.sed 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).
[Ill.u.s.tration: 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.
[Ill.u.s.tration: 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 cla.s.s, to which our attention will be confined chiefly, all include two princ.i.p.al parts--a metal receiver and a graduated gla.s.s 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 s.p.a.ced will the graduation marks of the measure be, and the more exact the readings obtained.
[Ill.u.s.tration: 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 gla.s.s catcher into which the funnel delivers the water for storage until the time when it will be measured in a graduated gla.s.s. 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 bra.s.s 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 gla.s.s tubing having an internal diameter not more than one-quarter that of the receiver This is to serve as measuring gla.s.s.
[Ill.u.s.tration: 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 pa.s.s 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.
[Ill.u.s.tration: 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 bra.s.s 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.
[Ill.u.s.tration: 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 compa.s.s 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.
[Ill.u.s.tration: FIG. 165--Wind vane with dial.]
The rod should preferably be of bra.s.s, which does not corrode like iron. If the uppermost 18 inches or so are of 1/4-inch diameter, and a.s.signed 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 bra.s.s at the points where the brackets are.
[Ill.u.s.tration: FIG. 156.--Elevation and plan of vane.]
The Bevel Gearing.--Two bra.s.s 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 bra.s.s) turns in a bra.s.s 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.
[Ill.u.s.tration: 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 pa.s.sing 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.