ANEMOMETER, FRom aveg,g, the wind, and wiTOV, a measure, is an in strument or machine for measuring the force and velocity of the wind: The direction of the wind is determined by the ancmoscope.
The first instillment that seems to have been inven ted for measuring the force of wind, was laid before the Royal Society in the year 1667, by Dr Croune, one of its most active members. A fan was included in a cylindrical vessel. The circumference of this vessel was divided into 52 or more equal parts, end slender slits, for admitting the air into the cylindrical box, were made at each of these divisions. The object of this in strument was to magnify the velocity of the air, by its passage through these small slits, and to measure its concentrated influence, by the effect produced upon the fan. This contrivance was not found to answer in prac tice.
As the preceding instrument had not the desired ef fect of rendering sensible small motions of the air, that were otherwise imperceptible, Dr Hooke attempted to remedy this defect, by constructing an instrument, so that the boxes and cells should collect as great a quan tity of air in motion as could enter a square aperture, equal to the greatest diameter of the instrument, and contract it into as small a passage as was desired. For this purpose he joined at their vertex a, Plate XXVIII. Fig. I. two hollow tin cones, a bed c, e f g h, and after dividing the limb of each into 32 equal parts, he inserted, at each division, a triangular partition, such asabf,a t a, a 12 g, a i k, a e g, E.Te. A small triangular portion was cut from the vertex a of each of these 32 partitions, so that a cylindrical hole, for containing a small vane or weather-cock, was left in the centre of the instrument. These partitions, or valves, were soldered between the surfaces of the two tin cones, as is shewn in the figure, so that the whole column of air which strikes the side of the cylinder f b edh g, is collected by the external apertures, and made to pass through the small narrow holes which form the little square at a. Hence it is obvious, that the real velocity of the wind will be to the velocity of the air in the small opening at a, as the area of these openings is to the area of the large apertures. " By this contrivance," says Dr Hooke, " it is easy to make a ball, at the top of a steeple, turret, &cc. that by the air's blowing of a pipe contrived in it, the quarter and strength of the wind may, at all times, either by night or day, be easily discovered." It will appear,
from the slightest attention to this anemometer, that, from the great number of triangular partitions, and the obliquity of their position to the direction of the wind, its velocity will be greatly retarded, and one uniform blast will not be maintained at the square opening a, where the various currents of air, moving in different directions, and with different velocities, must unite into one stream. This evil, however, may in a great mea sure be avoided, by using a single cone, whose sides form about an angle of 45' with its axis. This cone, being supported near its base upon a perpendicular arbor, will act as a vane, and will always turn its larger aperture to the wind. The velocity of the wind, when, concentrated at its vertex, may be measured by the num ber of revolutions performed in a given time, by a small wheel furnished with vanes.
The anemometer invented by Wolfius is represented in Plate XXVIII. Fig. 2. where A, B, C, D, are four „small sails like those of a wind-mill, forming angles of 72° with a vertical plane, perpendicular to the axis. On the horizontal axis to which these sails are fixed, is fitted an endless screw, EF, Fig. 3. that gives motion to a toothed wheel, GH. The axis of this wheel carries an arm LK, hollowed out to permit the leaden hall, L, to be placed at different distances from the centre, I, as circumstances may require. On the axis I, is also pla ced an index, M, Fig. 2. which indicates, on the gradua ted quadrant N, the angle to which the arm IK may be raised. The sails are turned to face the wind by the vane OPRQ, and a counterweight, Y, Fig. 4. is added to balance the arm IK, so that it. may be regarded as a Iine destitute of weight. When the sails are put in motion by the impulse of the wind, the axis EF, Z. upon which they are placed, turns the wheel GH, by means of the endless screw ED', and consequently raises the arm IK, with the leaden ball L. But as the ball rises, its distance from the centre of motion I, is augment ed, and it therefore opposes an increasing resistance to the power of the wind, till, at last, it completely balances it. In this situation, the angle formed with a vertical line by the arm IK, as indicated on the graduated arch, becomes a measure of the impulsive force of the wind. —Fig. 2. is a perspective view of this anemometer. Fig. 3. a section by a plane, parallel to the axis on which the sails are placed ; and Fig 4. a section perpendicular to the axis of the sails.