The quadrant for measuring the velocity of running water, seems to have first suggested the anemometers of Poleni and Pickering. In the same manner, the bent tube, tube recourbe, invented by Pitot for the same pur pose, appears to have been the origin of Dr Lind's wind-gage. This simple and ingenious instrument con sists of two glass-tubes AB, CD, about 9 inches long, having a bore of 4-10ths of an inch in diameter. These tubes are connected at their lower extremities by ano ther small tube of glass, with the bore of 1-10th of an inch. To the upper extremity of the tube AB, is fitted a thin metallic tube, F, bent at right angles, so that its mouth may receive horizontally the current of air. A quantity of water is poured in at the mouth, till the tubes are nearly half full, and a scale, HI, of inches and parts of an inch, is placed between the tubes. When the wind blows in at the mouth, the column of water is depressed in the tube AB, and elevated in the same degree in the other tube ; so that the distance between the surfaces of the fluid in each tube, is the length of a column of water, whose weight is equivalent to the force of the wind upon a surface equal to the base of the column of fluid. The little tube, ab, which connects the other two, is made with a small aperture, to prevent the oscillation of the fluid by irregular blasts of wind. The absolute velocity of the wind may be easily deduced from the height of the column of water, by a simple calculation, or may be found by inspection from the table at the end of the article. The undulations pro duced by sudden gusts of wind, would be still more completely prevented by making the small tube, which connects the two large ones, of such a length as to lie double between the other two, and be equal to the length of either. The same effect might also be produced by making a thin piece of wood float upon the surface of the fluid in each tube.
The musical anemometer of Delamanon, of which the first idea was suggested by Dr Hooke, is an instru ment more curious than useful. It is chiefly composed of twenty-one tubes, having their bores in such a pro portion, that the wind entering into each tube may give successively three octaves. The first, ut, ought to cor respond to the force of the wind acting upon a square foot of surface, and raising a weight of five ounces ; re, to 10 ounces, and in/ to five ounces. The notes of the second and the third octave will correspond with a weight which increases progressively by three ounces. By means of small plates, adjusted to springs, only one tube sounds at a time, and hence, the tube which opens itself, will, by this contrivance, shut all those which are below it. In order to judge of the force of the wind, we have only to listen to the sound which is emitted. If we hear, for example, the so/ of the first octave, the force of the wind will be 25 ounces, upon a square foot ; and if we hear the si of the second octave, the wind has become a tempest. Eight other tubes, with acute sounds, and directed towards eight different points of the com pass, are employed to indicate the direction of the wind. Hence, when the wind blows, two sounds are distinctly heard, one of them pointing out the force of the wind, and the other the quarter from which it proceeds.
An instrument for ascertaining the wind's velocity, has been recently described in the Lecons de Physique de L'Ecole Polytechnique, par Pujoulx. It is shewn in Plate XXIX, Fig. 10, where AA is the section of a plane surface exposed to the wind, and fixed to the horizontal arm C, which moves in the cylinder DD, which contains the bladder B, filled with air, This bladder is connected with the bent glass tube tit, containing a coloured fluid. When the wind blows against the surface AB, the cir cular plate E, presses against the bladder B, and, by compressing the included air, forces it up the bent tube, and raises the coloured liquor. When the pressure upon the bladder diminishes, by the abatement of the wind, the bladder will recover its former figure, and the liquor will descend in the tube. The graduation of this instrument should be effected experimentally, by load ing the surface AA with different weights in succes sion.
The instrument proposed by M. Bouvet, for measur ing the force of the wind at sea, is shewn in Plate XXIX. Fig. 11. It is composed of several small plates of cork, furnished with feathers, and so light as to follow the direction of the wind. These plates of cork are placed, as in the figure, upon a light rod which passes through their centre, and the velocity of the wind is determined by the space through which it flies in a given time.
The anemometer invented by Gusteau, and presented to the French Academy of Sciences, in 1777, does not seem to have any particular merit. At the top of a hol low cylinder, two or three feet long, is placed a ball, in the middle of which is a spring, whose force is ascer tained. Upon the ball, a plate of iron, about six inches high and nine long, is fixed vertically. One extremity of the spring is fastened to this plate, and the other ex tremity to a brass wire which passes down the cylinder, and carries an index that points out the force of the wind on a scale fixed to the cylinder. The graduation of this scale is effected in the common way, by loading the spring with different weights. The whole apparatus has a mo tion of rotation, by means of a vane, in order that the plate of iron may be perpendicular to the wind.
In the Observations, &c. par Rozier, for 1781, the che valier Dalberg has described an anemometer for finding the direction of the wind, its inclination, and its relative and absolute force. As the illustration of this compli cated apparatus requires two quarto plates, it cannot be expected that we should insert a description of it in this place. It has the merit neither of simplicity nor ingenu ity, and will most probably never be put in execution. The direction of the wind is determined in the common way, by an index, which moves along with the axis that carries the vane. The force of the wind is estimated from the weight which balances its pressure against a plane surface moving about a centre : and the inclina tion of the wind is ascertained by the inclination of a rod of sheet iron, which always lies parallel to the line of the wind's direction. To produce these effects, and render them visible in the observer's apartment, levers, and quadrants, and friction-wheels, and pulleys, and plumb-lines, are combined without judgment or inge nuity.