AN'EMOM'ETER (Gk. iiveenc, enemas, wind -1- metron, measure). An instrument used to measure the velocity of the wind, its pressure, or other effects produced by it. The first instrument of this kind is commonly known as Ilooke's pendulum anemometer, and is men tioned as early as 1667. It is, however, likely to have been the common product of the members of the first meteorological committee of the Roy al Society of London, among whom Hooke and Sir Christopher \Vren were prominent members. This form of instrument was revived in 1861 by l'rofessor H. Wild, and now used in Switzer land and Russia, where it is known as Wild's tablet anemometer. In this instrument a plane square tablet is suspended vertically from a horizontal axis which is kept by a wind-vane always at right angles to the direction of the wind; the tablet is raised by the wind to an in clined position of temporary rest, and its angular inclination to the vertical is noted on a grad uated arc ; circular plates, and especially spheres, have been sometimes used instead of the plate. About 1724 the use of a vertical pressure plate, having springs or weights at its back against which the plate is pushed by the wind, was introduced by Leupold; at the present time the pressure plate anemometer is used at a few European observatories in the form arranged by Osier for the British Association for the Advance ment of Science. Theoretically, the most per fect modification of Leupold's anemometer is that devised by Jelinek in 1850, in which the springs behind the pressure plate are inclosed in a cylin drical ease, which eliminates the action of the wind or the partial vacuum at the back of the plate. A third class of pressure-anemometers is that of Lind, in which the wind-pressure acts on the surface of a liquid in a U-shaped tube, raising it in one leg of the U and depress ing it in the other.
Various other forms of pressure-anemometers have occasionally been used by meteorologists, but at the present time the tendency is to aban don all these in favor of instruments that rotate and give more or less correctly the velocity of the wind. This tendency is justified by the consideration that in meteorology we need only the velocity of the wind, and by the fact that, al though the engineer needs to know the pressure of the wind against engineering structures, yet he cannot obtain this with sufficient accuracy from the pressures recorded by the small flat surfaces that are used in ordinary anemometers.
In fact, the pressure of the wind against an ob stacle depends not merely on the area of the transverse section of that obstacle, but on the shape of that section, and even still more on the longitudinal section in the direction of the wind. Thus, the pressure of the wind on wires, ropes, and rods is much greater than on globes of the same transverse section: the pressure on a tri angle is greater than that on a square or circle of the same area. In general, it is more impor tant to know the velocity of the average wind and of its maximum gusts than to know its pressure on some assumed arbitrary solid. When rain is driven with the wind, the combined pres sure due to both is needed in engineering studies.
Anemometers for measuring wind velocity include both the suction-anemometers and the rotation-anemomete•s. In the former the open end of a long, vertical tube is freely exposed to the wind in such a way that it blows as nearly as may he transverse to the axis of the tube. The end may he fitted into the side of a horizontal contracted tube as in Venturi's instrument; or may end conically in the air, or squarely, and without any adjunct. The passage of the wind across the open end of the tube produces a dimi nution of barometric pressure within it, which increases with the velocity. The exact measure ment of this depression gives the basis for com putation of the velocity of the wind. This prin ciple was known to the experimenters of the American Academy of Arts and Sciences (see their report for 1847), and to those of the Frank lin Institute (see their report of 1842), and is that which explains the draught up a good chim ney; but it was first applied to the measure ment of the wind in England by Fletcher in 1867. The modifications of Fletcher's anemometer made by Hagemann, of Denmark, and by Dines, of England, appear to be especially appropriate to the measurements of gusts. The combination of suction-anemometer. pressure-anemometer, and aneroid barometer recommended by Professor Cleveland Abbe in 1882, and especially the ap plication to the tube of parallel plates that en tirely annul the wind effects seem to be essential if we would determine the true barometric pres sure with a barometer exposed to the wind, as, for instance, on a mountain top.