A considerable improvement upon the preceding anemometer made by our ingenious countryman Mr Benjamin Martin, Plate XXVIII. Fig. 5. On the horizontal axis, QM, which carries the four sails, is fixed a wooden cone, MNO. 'When the sails are moved round by the impulse of tlic wind, the weight R, append ed to the extremity of the string RT, winds round the surface of the cone, from the vertex towards the base NO. As the string advances towards NO, it continues to act at the extremity of a longer lever, till it becomes an equilibrium to the force of the wind, and consequently a measure of that force. Another weight, S, suspended by a string on the opposite side of the axis, is a coun terpoise to the weight R, when the machine is at rest, and the string RT at the smaller end of the line. If the diameter of the cone, where the string lies when the machine is at rest, is to the diameter of its larger ex tremity NO, as 1 to 20, then a scale of twenty parts, formed on the surface of the cone, will indicate the force of the wind, when the string rests at any of these divi sions. This instrument would be rendered much more portable, by making the string coil round a spiral groove, formed upon the plane surface of a circular wooden wheel, and by placing the divisions of the scale upon the spiral line.
The subject of anemometers has received particular attention from 'the ingenious M. D'Ons-en-Bray, who has proposed no fewer than fire of these instruments : 1. A pendulum anemometer ; 2. A lever anemometer, which ascertains the relative force of the wind ; 3. A fusee anemometer, for determining its absolute force ; 4. A steelyard anemometer, for weighing the absolute force of the wind, or ascertaining its impulsive force against a surface a foot square ; 4. An anemometer for determining, on board a ship, the velocity or force of the wind against the sails. The pendulum anemometer is the only one of these instruments of which he seems to have published a description. It is a complete, though complicated instrument, as it marks upon paper, in the absence of the observer, not only all the winds which have blown during 24 hours, but also the time when they began and ceased to blow, and their different velocities and relative intensities. In a work so limited as ours, it would be injudicious to give a description of a machine which requires no fewer than six folding plates for its illustration, and which is not remarkable for the application of any new principle. By com plicated machinery, almost any mechanical effect can be produced : But it is only when the machine is founded on some new principle, or distinguished by simplicity of construction, and a judicious combination of its parts, that it is worthy of being handed down to posterity. A complete description of D'Ons-en-Bray's anemometer will be found in the Memoirs of the French Academy for 1734, 8vo. part 1. p. 169.
In the anemometer invented by the learned M. Bouguer, Plate XXVIII. Fig. 6. the velocity of the wind is deduced from the effect which it produces in the com pression of a spiral spring. This spring is included in the hollow cylinder AB, the anterior part of which ha, a quadrangular aperture, op, and its posterior part ter minates in a solid piece, D, which can be moved in the tube AB, and fixed by a screw, B, in any position. The
part IH, of the axis IG, is quadrangular to the circular disc H ; but its posterior part HG, is cylindrical, and moves freely in the hollow cylinder 1', cut out of the solid piece D. At the extremity, I, of the axis, a plane surface, of which KEL is a section, is fixed by means of a screw, s; n is a friction wheel, on which the lower side of the axis III rolls ; and nit is a delicate spring, furn ished with an index, in, made of white lead for marking on the upper side of the axis IH, the effect produced by the action of the wind. When the plane surface KEL, which is generally made a foot square, is exposed to the wind, the spiral spring CC, will be compressed against the stop qr, as in the common spring steelyard, and the surface KEL, will be brought nearer the index in, which will point out, on the upper surface of the axis IH, the pressure, in pounds and ounces, which the wind exerts upon a square foot of surface, from which its velocity is easily deduced. The scale is made by putting the machine in a vertical position, and marking the place on the axis to which the index points, when the plain surface is loaded with different weights in succes sion.
The preceding anemometer has been ingeniously combined with the apparatus employed by Smeaton is his Experiments on Windmills, by M. Zeiher of the Imperial Academy of St Petersbu•gh, in order to deter mine the absolute velocity of the wind.* The two in struments combined are represented in Plate XXVIII.
Fig. 7, where 00 is Bouguer's anemometer, fixed at the extremity of the lever FG, so that the axis RS of the anemometer is perpendicular to the lever. It is obvious, by inspecting the machine, that when the winch \V is turned, the rope T acts upon the barrel H, of the upright axis D, and consequently gives a rotatory mo tion to the line FG, with its counterpoise N. The pen dulum LM, consisting of two metallic balls, performs two oscillations during one complete revolution of the lever FG. When this apparatus is placed in a room where the air is completely tranquil, and a rotatory mo tion given to the lever, the spring of the anemometer will be compressed in consequence of the impulse of the surface G, against the air ; which is exactly the same as if the air had moved with the velocity given to G, against the surface G at rest. The velocity of G be ing determined by the oscillations of the pendulum, the corresponding compression of the spring CC is marked upon the upper surface of the axis 11-I. By giving dif ferent velocities to G, we obtain the compression of the spring which corresponds to them, and therefore deter mine the absolute velocities which answer to the dif ferent divisons of the scale. When this instrument is applied to use, there is considerable difficulty in ascer taining the division of the scale to which the index points, as, from the inequality in the blast of wind, the instru ment is continually agitated. This evil, however, has been in a great degree, remedied in the following ane mometer, invented by M. Brequin Demenge.