RAIN-GAUGE, a vessel for measuring the quantity of rain which falls on any particular part of the earth's surface, the quantity being indicated by the depth of the precipitated water which would cover the ground about the spot, supposing the ground to be horizontal and that the water could neither flow off nor penetrate into the soil.
In order to ascertain the quantity of rain whicb has fallen during the continuance of a shower, it might suffice to place a prismatical or cylindrical vessel, open at the top, in a horizontal position on the ground or on the top of a building, and, when the shower has ceased, to measure the depth of the water in the vessel by a scale of inches. But, unless the depth were ascertained immediately, a portion of the water would be carried off from such a vessel by evaporation, and the measure would be less than it ought to be. The difficulty also of ascertaining the true amount of a small depth of water would render the instrument of no practical use. For the purpose therefore of obtaining a more correct estimate of the quantity of rain, it has always been the practice to receive the water in a second vessel, or in a tube, the area of whose horizontal section is less than that of the first, so that the height of the column may be greater. And, since the heights of equal quantities of water in two prismatical or cylindrical vessels are inversely proportional to their bases, it is easy to perceive how a rod may be graduated so as to show, in inches, the depth of water in the upper vessel, and consequently the depth which would have lain on the ground if no absorption had taken place.
Originally this instrument, which has been called indifferently odometer (11800p and Actrpop), pluviometer (pluvia), and ombrometer (EAPpos, rain), was nothing more than a prismatical box, having a square base, open at the top and communicating with a prismatical box, placed vertically under it, by means of a pipe open at both ends; the area of a horizontal section of the lower box being, for the reason above given, less than that of the upper box. But it is evident that a prismatical or cylindrical vessel must retain, by adhesion to its sides and bottom, a sensible portion of the water which enters it ; and consequently the depth measured in such vessel must indicate a quan tity of rain less than that which has really fallen ; it has therefore been customary of late to make the upper part of the vessel in the form of a funnel, or inverted cone.
The most general construction of a rain-gauge is shown in the sub joined diagram, which represents a vertical section of the instrument.
The part CD E is a conical funnel, open both at top and bottom, and the lower extremity enters into the cylinder ir G below, which thus receives the rain from the funnel. The rod AB passes through a perforation in a bar c D (in the direction of a diara,ter of the cone at its upper surface), and is attached, at B, to a circular piston, which has nearly the same diameter as the interior of the cylinder : the weight of the piston and rod is such as to allow the former to float with its upper surface on a level with the surface of the water; and the graduations, which are numbered towards B, commence from a point a on a level with the upper surface of the bar C D, when the piston B touches the bottom of the cylinder. A rim, of a cylindrical form, rises a little way above the upper extremity of the conical part of the funnel, in order to prevent the rain-water, which would strike the interior of the latter near that extremity, from being thrown out in consequence of the shock.
The diameter of the funnel at the top may be 12 inches, and that of the cylinder 6 inches; in which case the area of the horizontal section on which the rain falls will be to that of the cylinder in the ratio of four to one. Hence a depth of water equal to one inch at the horizontal section will be expressed by a space equal to four inches on the length of tho rod ; and, each of such spaces being divided into 100 parts, the depth of water at the said section will be indicated in hundredths of an inch. The height of the cylindrical vessel below the funnel may be from 25 to 30 inches.