Rain-Gauge
This is an instrument for measuring the amount of rain that falls in any place in any given time. Various forms are used, but all are much alike in essential points. In all there must be a collecting surface of known size. This is generally a brass funnel, with an upright rim about 1 1/2in. high around it, to prevent the drops from splashing over the edge. The rainwater runs down a tube into a receiver, which is protected from any water getting into it, except by the tube, and prevents the water in it from evaporating. At certain hours, the amount of water in the amount of water in the receiver is poured into a graduated vessel, and is carefully measured. Each mark on the measure represents, usually, 1/100 in. of rainfall. The receiver is generally made to hold 3in. depth of rainfall. The collecting funnel may be of any size, so long as accurately known; but 5in. is the common diameter. The funnel must be placed absolutely horizontal, and should stand in the middle of an open piece of ground, in order to avoid eddies. The rim should be at least 6in. above the surface on which it stands; but it ought not to be raised much, as, other conditions remaining the same, the higher the gauge stands, the less rain it catches. Snow must be melted, and measured as water, in calculating the total annual rainfall. Very slight showers cannot be measured, as the rain-drops evaporate from the funnel without running into the receiver; hence, the record may be rather low, if such showers are frequent.