PLUVIAMETER

, a machine for measuring the quantity of rain that falls. There is described in the Philos. Trans. (numb. 473, or Abridg. x. 456), by Robert Pickering, under the name of an Ombrameter,| an instrument of this kind. It consists of a tin funnel d, whose surface is an inch square (fig. 6, plate xx); a flat board aa; and a glass tube bb, set into the middle of it in a groove; and an index with divisions cc; the board and tube being of any length at pleasure. The bore of the tube is about half an inch, which Mr. Pickering says is the best size. The machine is sixed in some free and open place, as the top of the house, &c.

The Rain-gage employed at the house of the Royal Society is described by Mr. Cavendish, in the Philos. Trans. for 1776, p. 384. The vessel which receives the rain is a conical funnel, strengthened at the top by a brass ring, 12 inches in diameter. The sides of the funnel and inner lip of the brass ring are inclined to the horizon, in an angle of above 65°; and the outer lip in an angle of above 50°; which are such degrees of steepness, that there seems no probability either that any rain which falls within the funnel, or on the inner lip of the ring, shall dash out, or that any which falls on the outer lip shall dash into the funnel. The annexed figure is a vertical section of the funnel, ABC and abc being the brass ring, BA and ba the inner lip, and BC and bc the outer.

Note, that in fixing Pluviameters care should be taken that the rain may have free access to them, without being impeded or overshaded by buildings, &c; and therefore the tops of houses are mostly to be preferred. Also when the quantities of rain collected in them, at different places, are compared together, the instruments ought to be fixed at the same height above the ground at both places; because at different heights the quantities are always different, even in the same place. And hence also, any register or account of rain in the Pluviameter, ought to be accompanied with a note of the height above the ground the instrument is placed at. See Quantity of Rain.