RAIN GAUGE. An apparatus to catch the rain and measure its equivalent depth on the ground at any place. The simplest rain gauge consists of a vertical cylinder, into which one may dip a graduated 'tick and read off the depth of the collected water. Such an instru ment is furnished by the United States Weather Bureau to its voluntary observers, and is illus trated in the accompanying figure. Other gauges consist essentially of funnels to catch the water, glass bottles to hold it. and slender grad uated glass tubes by which to make minute TS' as an axis, it will be seen that colored hands of light will be seen by the eye at T of an angular diameter greater than any in the pri mary bow. and with the violet having the largest and the red the smallest diameter. The angular radius of the red is about 50° 24' and of the violet 53° 22'. The values of the diameters measurements. Self-registering gauges are now made so economically that they are always to be recommended: those of the simplest pattern em ploy a tipping bucket, divided into two equal compartments, and tipping to the right and left alternately, according as the right or left com partment is filled. Each tip corresponds to a definite depth. such as a hundredth of an inch. Another favorite form consists essentially of a weighing machine. and every ounce of rain. or the equivalent depth, is properly recorded on a. sheet of paper moved by clockwork. Both forms. as perfected by Professor C. F. Marvin, are used by the United States Weather Bureau.
It has long been known (since Tleberden. 1776) that perfectly reliable gauges collect less rain the greater their height above ground. and it is now satisfactorily demonstrated that this is entirely due to the fact that the gauge causes eddies of wind around and even within its mouth. These eddies carry away the smaller
particles of rain or snow to a greater or less extent. On the average of all records that have been made in Europe and America and in all kinds of rain, the deficit of rain recorded by the gauge has been found to increase in proportion to the square root of the altitude above ground. Consequently time correct rainfall can be approxi mately determined if the observer notes the dif ference between the catches of two or more sim ilar gauges set at different altitudes above the ”round.
In 1855 Joseph Henry. in 1875 F. E. Nipher, and in 1886 Wild and Boernstein described ap pliances that largely annul the error due to the wind. Henry surrounded his gauge with a thin broad rim near the top, either horizontal or sloping downward; Nipher surrounded it with an umbelliform structure of wire gauze; Wild surrounded it with a closed board fence, but Boernstein placed a simple open board fence at a distance of about two feet around the gauge. When a gauge is set up within a shallow saucer like depression in the ground. so that the mouth of the gauge is level with the earth beyond the edges of the pit, or when it is placed on the lint roof of a building and protected on all sides by a high balustrade, good measurements are also obtained. Most elaborate investigations into the peculiarities of rain gauges are published in the annual volumes of Symons's British Rainfall. For other special studies, see the works of Eastman, United States Naval Observatory: Hellmann, Berlin, 1890; Abbe, Washington, 1887 and 1S92; Boernstein. Saint Petersburg, 1S90; E. Berg, Saint Petersburg, 1895.