RAIN-GAUGE, or Pluviometer, is 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. The instrument generally consists of a conical funnel, open both at top and bottom, the lower extremity entering into a cylinder below, which thus receives the rain from the funnel. A graduated rod passes through a perforation in a bar fixed in the direction of a diameter of the cone at its upper surface, and is attached, at the lower extremity, to a circular piston, which has nearlythe 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. A rim, of a cylin drical 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 ex tremity, 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 sec tion 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 4 inches on the length of the 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.
For the sake of diminishing the evaporation and of measuring small quantities of rain with greater precision, the diameter of the cylinder is sometimes reduced to 2 inches, and the collected water is, by means of a small pipe, inserted in the bottom of the cylinder, and furnished with a cock made to pass into a glass tube whose interior diameter is half an inch. In this case the diameter of the upper extremity of the funnel being the same, a shower of rain whose depth on the ground might be one-hundredth part of an inch, would be indicated by 5.76 inches in the tube.