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.