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.
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