ANEMOMETER (Gr. ancinns, the wind,. and metron, a measure; Fr. Ger. Windmesser), an instrun.ent for measuring the strength and velocity of the wind. The simplest and best A. is '.hat which is generally known as Robinson's hemispherical-cup A. It consists of four hollow hemispheres or cups fixed to the ends of two horizontal iron rods cross,ng each other at right angles, and supported on a vertical axis which turns freely. The cups revoive.with a third of the wind's velocity, and the instrument is so constructed that 500 revolutions are made whilst a mile of winds passes over it. The revolutions are registered by a system of wheels similar to those of an ordinary gas meter. The difference between two readings gives the number of revolutions passed over during the intervening time, from which the miles can be calculated, and the rate per hour.
The following table gives approximately the relation of the height of the water in the A. to the force and velocity of the wind in winds of different characters. (See AERODY NlalICS.) Height of Pressure per Velocity water, square foot. per hour.
Feeble wind 4- in. lbs. Fresh breeze ...... " " 16} Very strong wind.... .. 1 " " 82i Tempest . .... . ...... 4 " 20" Pressure anemometers are of very great importance in meteorological observatories.
Of these, the most complete is that invented by Osier. In this instrument, the force of the wind is ascertained in a different way from the hemispherical-cup A. A brass plate one foot square is suspended by means of springs, and being attached to the vane of the instrument, is maintained at right angles to the direction of the wind. This plate, by the action of the wind, is beaten back upon the, springs, and in so doing, causes a pencil to move backwards and forwards on a sheet of paper placed below it. This sheet of paper is made to pass under the pencil in a direction at right angles to its oscillation; and by means of clock-work, moves at a uniform rate, so that the force of the wind at any particular time of the day is recorded with perfect accuracy, A pencil in connec t? am 2 4 5 . 9 o I i tion with the vane, and moving in the same transverse line as the former, records the changes in the direction of 1=1111111111111111M31:1321313:111MMINM the wind; arid a third pencil, guided mmi SI by a rain-gauge, registers the quantity Mum imMIIIIMIIMINII=IM