RAIN-GAIIGE, The use of rain-gauges is to ascertain the amount of rain which falls at any given place. They are of various constructions. The simplest is that which con sists of a metallic cylinder, from the bottom of which, a glass tube, divided into inches and parts of an inch, projects downward. It is provided with a funnel, inserted within at the top, to prevent evaporation, and the rain-water is emptied out by means of a stop cock at the bottom, or, still simpler, by a hole pierced in the funnel at the top. As this form of gauge is objectionable on account of the frequent breakage of the glass.tube by frost, a floatis used instead, which is raised by the water, and a scale is attached to it, to show the quantity of rain received. As this gauge does not admit of very nice readings, another sort is frequently employed, viz., a receiving-vessel and a glass measure of much smaller diameter, which thus admits of as nice graduation as may be desired. As, practically, there is often great difficulty or trouble experienced in replacing the glass measure when it chances to get broken, the late G. V. Jagga Rao, a wealthy zemindar of Vizagapatam, proposed a gauge in the form of a funnel having a diameter of 4.697in., or an area of 17.33 sq.inches. Now, as a fluid ounce contains 1.733 cubic iu., it follows that for every fluid ounce collected by this gauge, the tenth of an inch of rain has fallen. This measure can of course, be graduated to any degree of nicety, and may be repro. duced at pleasure. It has also the great merit of being by far the cheapest gauge, cost ing only 4s. 6d. -Self-registering rain-gauges have been invented by Osler, C'rosley, and Beckly, but they are too expensive to come into common use.
A most important point with regard to the rain-gauge is its height above the ground.
Prof. Phillips found the fall of rain at York for 12 months in 1833-34, to be 14.96 in. at a height of 213 ft. from the ground; 19.85 in. at 44 ft.; and 25.71 in. on the ground. This remarkable fact—viz., that different quantities are collected at different heights, the amount being always greater at the lower level, has been confirmed wherever the experiment has been made. No perfectly satisfactory account has yet been given of this singular phenomenon. The condensing of the vapor of the atmosphere on the sur face of raindrops as they fall—the rebound of the finer particles into which many of the drops break themselves as they strike with violence on the ground—and the eddies and currents which prevail most and strongest around isolated objects raised above the sur face of the ground, to a large extent account for the phenomenon. Of these three, the greatest weight is to be given to the last two; and this is confirmed by the fact, that a gauge placed on the roof of a building that happens to be flat, of considerable area, and with few or no chimney-Stalks to disturb the air-currents, collects an amount equal to that collected at the same place by a gauge on the ground. The proper size and shape of the rain-gauge, and its height above the ground, so as to measure with the greatest exactness possible the real quantity of rain that falls, about all of which much diversity of opinion exists, have been ably navestigated by series of extensive experiments con ducted by maj. Ward, Mr. Symons, rev. Charles Griffith, and others, and the results have been published annually in Symons's British Rainfall.