ANEMOMETER

, an instrument for measuring the force of the wind.

An instrument of this sort, it seems, was first invented by Wolfius in the year 1708, and first published in his Areometry in 1709, also in the Acta Eruditorum of the same year; afterwards in his Mathematical Dictionary, and in his Elem. Matheseos. He says he tried the goodness of it, and observes that the internal struc- | ture may be preserved, so as to measure the force of running water, or that of men or horses when they draw or pull. The machine consists of sails, A, B, C, like those of a wind-mill, against which the wind blows, and by turning them about, raises an arm K with a weight L upon it, to different angles of elevation, shewn by the index M, according to the force of the wind. (Plate III. fig. 3)

In the Philos. Trans. another anemometer is described, in which the wind being supposed to blow directly against a flat side, or board, which moves along the graduated arch of a quadrant, the number of degrees it advances shews the comparative forceof the wind.

In the same Transactions, for the year 1766, Mr. Alex. Brice describes a method, successfully practised by him, of measuring the velocity of the wind, by means of that of the shadow of clouds passing over a plane upon the earth.

Also in the same Transactions, for the year 1775, Dr. Lind gives a description of a very ingenious portable Wind-Gauge, by which the force of the wind is easily measured; a brief description of the principal parts of which here follows. This simple instrument consists of two glass tubes, AB, CD, (Plate III. fig. 4.) which should not be less than 8 or 9 inches long, the bore of each being about 4/10 of an inch diameter, and connected together by a small bent glass tube ab, only of about 1/10 of an inch diameter, to check the undulations of the water caused by a sudden gust of wind. On the upper end of the leg AB is fitted a thin metal tube, which is bent perpendicularly outwards, and having its mouth open to receive the wind blowing horizoutally into it. The two tubes, or rather the two branches of the tube, are connected to a steel spindle KL by slips of brass near the top and bottom, by the sockets of which at e and f the whole instrument turns easily about the spindle, which is fixed into a block by a screw in its bottom, by the wind blowing in at the orifice at F. When the instrument is used, a quantity of water is poured in, till the tubes are about half full; then exposing the instrument to the wind, by blowing in at the orifice F, it forces the water down lower in the tube AB, and raises it so much higher in the other tube; and the distance between the surfaces of the water in the two tubes, estimated by a scale of inches and parts HI, placed by the sides of the tubes, will be the height of a column of water whose weight is equal to the force or momentum of the wind blowing or striking against an equal base. And as a cubic foot of water weighs 1000 ounces, or 62 1/2 pounds, the 12th part of which is 5 5/24 or 5 1/5 pounds nearly, therefore for every inch the surface of the water is raised, the force of the wind will be equal to so many times 5 1/5 pounds on a square foot. Thus, suppose the water stand 3 inches higher in the one tube, than in the other; then 3 times 5 1/5 or 15 3/5 pounds is equal to the pressure or force of the wind on the surface of a foot square.

This instrument of Dr. Lind's, measures only the force or momentum of the wind, but not its velocity. However the velocity of the wind may be deduced from its force so obtained, by help of some experiments performed by me at the Royal Military Academy, in the years 1786, 1787, and 1788; from which experiments it appears that a plane sursace of a square foot suffers a resistance of 12 ounces from the wind, when blowing with a velocity of 20 feet per second; and that the sorce is nearly as the square of the velocity. Hence then, taking the force of 15 2/5 pounds, above found for the force of the wind when it sustains 3 inches of water, and taking the square roots of the forces, it will be, as √12 : √15 3/5 :: 20 : 22 4/5 the 4th proportional, that is a velocity of 22 4/5 feet per second, or 15 1/2 miles per hour, is the rate or velocity at which the wind blows, when it raises the water 3 inches higher in the one tube than the other. And farther, as the said height is as the force, and the force as the square of the velocity, we shall have the force and velocity, corresponding to several heights of the water in the one tube, above that in the other, as in the following table.

In one instance Dr. Lind found that the force of the wind was such as to be equal 34 9/10 pounds, on a square foot; and this by proportion, in the following table, will be found to answer to a velocity of 23 1/4 miles per hour.

Mr. Leutmann improved upon Wolfius's anemometer, by placing the sails horizontal, instead of vertical, which are easier to move, and turn what way soever the wind blows.

Mr. Benjamin Martin also (Plate III. fig. 5) improved upon the same. He made the axis like the fusee of a watch, having a cord winding upon it, with two weights at the ends which make always a balance to the force of the wind on the sails. See his Philos. Britan.

And M. D'Ons-en-Bray invented a new anemometer, which of itself expresses on paper, not only the several winds that have blown during the space of 24 hours, and at what hour each began and ended, but also the different strength or velocity of each. See Mem. Acad. Scienc. an. 1734. See also the article Wind-Gauge.