ANEMOWETER, or wind-measurer, is an instrument for ascertaining the force of the wind, generally by finding the mechanical effect which it produces on the apparatus. Wolfs anemometer consists of four small sails, like those of a windmill, which turn on a horizontal axis : this axis is connected by wheel-work with another in which is inserted one end of a bar carrying a weight. The wind acting upon the sails causes this bar to turn in a vertical plane ; and, when it is in such a position that the weight on it counterbalances the pressure of the wind, the angle which it makes with a vertical line passing through the axis affords a measure of the wind's force.
Lind's anemometer consists of two tubes of glass, each 9 inches long and- inch diameter, which are open at their upper extremities, and are connected at bottom by a bent tube only diameter. These are partly filled with water, and the wind acting at the open end of one tube depresses the water in that tube and elevates it in the other, until the difference of height becomes a counterpoise to the pressure of the wind. This difference can ba ascertained by a graduated scale, and hence may be obtained the dimensions of a column of water whose weight is equivalent to the force of the wind. The velocity may thence be found, observing that the velocities are nearly as the square roots of the resis tances, and that on a square foot, the pressure of wind moving with a velocity of 20 feet per second, is 12 ounces.
In Regnier's anemometer, a her, carrying a wooden plate at right angles to it, protrudes from a box, through a hole, in the front of which it slides. This bar is met by a spring, which resists its further entry, until force is applied against the plate. In the interior of the box, the under side of the bar carries rackwork, which plays on a cog-wheel, the axis of which, passing through a side of the box, carries a hand round a dial-plate. The anterior surface of the wooden plate is pre sented to the wind, which presses upon it and forces back the bar, carrying the cog-wheel and hand through an angle, greater or less, according to the greater or less impulse of the wind.
Mr. Elliott, an overman at Pensher Colliery, invented an anemometer in 1835, to measure the rate of the currents of air in coal mines, with a view to regulate ventilation. It con sists of a case 12 inches high, 12 broad, and 4i deep. On a dial are two concentric circles ;
the inner circle being divided into 48 parts, and the outer into 60. The vanes are con nected with two indices or pointers, whose axes of motion are in the centres of the cir cles, like the hands of a watch; the one point ing to the outer circle and the other to the inner. The movement of one index has a certain ratio of velocity to that of the other ; and by the aid of a calculated table, the num ber of rotations of the slower index within a given time is made to determine the velocity with which the air is moving in or past the machine.
Mr. Biram, in 1843, invented an anemo meter for a similar purpose. It consists of a wheel one foot in diameter, having ten oblique vanes of thin sheet copper, perforated for lightness, and covered with tissue paper or varnished silk. The vanes are set at such an angle, that the wheel makes one revolution in the time a current of air two feet in length is passing through it. The axle of the wheel carries an endless screw, into which.works a wheel having thirty teeth ; and an index is fixed on the axle. As each revolution of the wheel is effected by the action of a current or column of air, two feet in length, oqe revolu tion of the index will register the passage of 60 feet of air.
Dr. 1VhewelPs anemometer differs some what from others in being calculated to mea sure the rate of the wind rather than its force. It gives no absolute measure of velocity in miles per hour ; and the rate is relative only to that of any one instrument at different times, not to that of any common or general standard. At the meeting of the British As sociation in 1816, Dr. Robinson described an alteration which he had effected in this instru ment. On a vertical axis are mounted three or four arms, carrying hemispherical cups at their extremities. These cups oppose much less resistance to air acting on the concave sides than on their convexities, and in such ratio that uniform revolution is produced at the rate of one-third of the velocity of the wind. From this measure, which might be made the same for all sizes of the instrument and in all places, the mean velocity of the wind during a given period could always be obtained in miles per hour. It is believed that this principle of uniform adjustment might be usefully applied to many practical purposes, such as the regulation of wind and water-wheels.