The primary factor in all kinds of weather is the moisture which is always present in varying quantity in the atmosphere. The second factor - the pressure of the air - depends upon the first. The average weight of the atmosphere is 14.7 lb. per square inch. The weight increases as the atmosphere becomes drier. This is easily tested by weighing a flask filled with dry air, and comparing the result with that obtained when the same flask is filled with a mixture of steam and air. The introduction of moisture will be found to reduce the weight. So it is with the atmosphere. When the weather is clear and the sky bright, there is very little water vapor mingled with the air; but when the sky is covered with clouds and wet weather is approaching, the air is laden with water particles, and is therefore lighter than usual.
It is here purposed to show the application of the above principle to the making of several kinds of weather indicators. The susceptibility of catgut to moisture is well known. This substance possesses the power of shortening its length and twisting when exposed to damp; and this twisting motion can be utilized in making a simple weather indicator that will provide amusement as well as information. It takes the form of a quaint little house with two doors, in one of which stands a woman and in the other a man. The latter comes outside when bad weather is threatened, and the woman retires, thereverse taking place when the weather is fine.
This effect is produced by means of a short piece oi catgut suspended from the roof by one end and attached at the other to the moving framework below. The two figures are glued to the ends of a short wooden bar, in the center of which an upright piece is secured. The lower end of the catgut is then attached to the upright. Damp air causes the catgut to twist in one direction, while dry air causes it to twist it the other, thus turning the framework on a pivot and changing the position of the figures. The figures are arranged according to the direction of this twist which must be tested by experiment.
The property of catgut in altering its length when exposed to damp is utilized in another form of weather indicator, made as follows: Take a baseboard about 1 ft. square and 1 in. thick, and fit to the back of it two banging plates. Mark two lines across the face of the board, one at each end, and about 1 1/4 in. from the edge. These lines must be drawn so that they will be horizontal when the baseboard is hung in position. At a distance of 1 1/2 in. from the side edge, measuring along the top line, insert a screw-eye, such as is used for picture hanging, having a ring about 1/4 in. in diameter. Mark a dot 1 1/2 in. from the screw-eye, and four more along the remainder of the line, leaving 1 1/2 in. space between them. Now, measuring from the side edge as before, mark a dot on the bottom line 2 1/4 in. from the edge, and finish off the line with four more dots 1 1/2 in. apart.
Now get ten little brass pulleys, about 3/4 in. in diam eter (these are used in Venetian blind work, and can be obtained at a hardware store), and the same number of button-headed screws, about 1 in. long, with shanks an easy running fit for the holes in the pulleys. Then insert a button-headed screw carrying a pulley at each dot in the top line - five in all. Screw a pulley the same number in the lower line, where the position of the pulleys falls between that of the pulleys in the top line. About 9 ft. of 1-16 in. catgut, as used for suspending clock weights, will be required. Tie one end securely to the screw-eye in the top row, and take a turn under the first pulley of the bottom row; then up and over the top row of the first pulley, and so on. If the pulleys have been properly spaced, as directed, a free length of catgut will hang from the last pulley of the top row.
A lead pointer must now be made. Take a piece of heavy sheet lead and bend it double; place apiece of wire in the fold and close up with a hammer. From this cut a fairly long triangle, leaving the wired doubled edge for the base; the two remaining sides should be equal and should come to a point. Remove the wire from the bend and in its place pass through the free end of the catgut, which should be so tied as to bring the point of the weight about halfway down the board, the surplus catgut being cut off.
All that now remains to be done is to paste on the scale, which should reach from the last pulley of the top row to the bottom of the board. After fitting together it is advisable to take off the pulleys, etc., and give the board, including the scale, a coat of varnish. It would also be an improvement if the edges of the board were chamfered. When dry, reassemble the parts, taking care not to screw in the button heads so far as to cause the pulleys to jam.
The action of the indicator is very simple. Moisture in the air causes the gut to contract, which is at once indicated by the rising of the lead pointer, a sure sign of wet weather. In fine, dry weather the gut lengthens, and the slack is drawn down by the pointer.
The two indicators described, however, rely for their action upon the moisture of the air, and are not affected by atmospheric pressure. Another variety now to be described is more truly oarometric and will, if properly read, indicate wind as well as rain. The general principle of this class of indicator depends on the fact that the volume of air enclosed in an elastic vessel varies in proportion to the pressure of the outer air.
An instrument of this kind can be made from an old electric incandescent bath. Get a burnt-out lamp of fair size, and have at hand a fine-cut file and a basin of water. Holding the bulb so that the pointed end is under the water in the basin, file away the point, still keeping it under water. Very 60on a jet of water will be seen rising into the globe. When this occurs, stop filing, but on no account lift the point out of the water. Hold the lamp steady until it contains enough water to weigh it down below the surface. Leave it there for about half an hour; then take it out, dry it, and hang it up, point downwards, by a string tied round under the cap. Wet weather will bring a drop of water of varying size to the outside of the point; in dry weather the glass will be quite dry. The cause of this action is that the gas which is always present in an old lamp bulb expands and contracts according to the atmospheric pressure. This expansion and con-tracton reacts upon the water, and the drop is exuded and withdrawn as the weather is wet or dry.
Another instrument of the same class is made from a tall, narrow bottle, a length of glass fine-bore tubing, some colored liquid, a cork to fit the bottle, and a strip of cardboard. After thoroughly cleaning the bottle, fit the cork, and when this has been adjusted, bore a hole centrally through the cork of a size to enable the tubing to fit tight. The tube, upon the fineness of whose bore depends the delicacy of the reading, should be about twice the length of the bottle. Bend the tube U-shape, by heating in a lamp or gas flame, so that the limbs are about 1 in. apart, and one limb about 2 in. longer than the other. Cut a piece of pasteboard to the same length as the short limb, and about 1 1/2 in. broad. Draw two lines centrally down the length, about 1 1/4 in. apart, and graduate a scale between them, from about 1 in. from the bottom to the same distance from the top.
Now push the long limb of the tube through the hole in the cork until the short limb is almost touching the under side; then secure the scale to the tube with a few stitches of white thread passing through the pasteboard and round the tube. This done, pour a colored liquid into the tube till it reaches about half-way up the scale in each limb. Now push the cork firmly into place in the neck of the bottle, with the tube and scale inside. Melt some sealing-wax and carefully spread it all over the surface of the cork and round the edge, as well as round the tube, to make it air-tight. On the approach of bad weather the colored liquid will rise in the long limb, because the air pressure outside will be slightly weaker than normal. For fine weather the liquid will sink in the long limb, the air being heavier. The action is explained by the fact that the bottle contains a fairly large volume of air, whose volume will vary more or less perceptibly with the variation of the atmospheric pressure.- "Work," London.