RAIN-GAUGE, also called OMBRO METER, UDOMETER, and PLUVIA METER. An instrument for measuring or gauging the quantity of rain which falls at a given place.
" The rain-gauge may be of very sim ple construction. A cubical box of strong tin or zinc, exactly 10 inches by the side, open above, receives at an inch below its edge a funnel, sloping to a small hole in the centre. On one of the lateral edges of the box, close to the top of the cavity, is soldered a short pipe, in which a cork is fitted. The whole should be well paint ed. The water which enters this gauge is poured through the short tube into a cylindrical glass vessel, graduated to cubit) inches and fifths of cubic inches. Hence, one inch depth of rain in the gauge will be measured by 100 inches of the gradu ated vessel, and 1-100th inch of rain may be very easily read off.
"It is very much to be desired that, being of such easy construction, more than one of these gauges should be erect ed ; or at least one placed with its edge nearly level with the ground, and another upon the top of the highest building, rock, or tree in the immediate vicinity of the place of observation, the height of which must be carefully determined, it having been satisfactorfiy ascertained that the height of the gauge above the ground is a very material element of the quantity of rain which enters it. The quantity of
water should be daily measured and reg istered at 9 A. :kr." A convenient form of the instrument is represented in the annexed figure, where the rain which enters the funnel is collected in a cylin drical vessel of cop per, connected with which at the lower part is a glass tube with an attached scale. The water stands at the same height in the cylinder and glass tube, and being visible in the latter, the height is read immediately on the scale; and the cylinder and tube be ing constructed so that the sum of the areas of their sections is a given part, for instance a tenth of the area of the fun nel at its orifice, each inch of water in the tube is equivalent to the tenth of an inch of water entering the mouth of the funnel. A stop-cock is added, by which the water is drawn off when the obser vation is made.