It is well known that when water is exposed to the wind, the quantity evaporated in a given time is pro portional to the velocity of the wind, the capacity of the air for moisture remaining the same. If, therefore, we expose to the wind a plane surface A, Plate XXIX. Fig. 15, consisting of sponge or coarse flannel, stretched across a metallic frame and saturated with water, and observe the quantity evaporated in a given time, we have a measure of the wind's velocity. The square surface A, is fixed at the end of the lever BCD, 11107 ing round C, as a centre, and the loss of weight is as certained by the weight E moving along the arm CD. The plane surface is turned to the wind by the vane G. This instrument furnishes us also with the means of de termining the sum of the velocities of the wind during any given period.
Another anemometer, founded on a different princi ple, is represented, in Plate XXIX. Fig. 16, where A is a metallic cap fixed upon the glass tube BE. This tube contains a fluid, whose specific gravity is greater than that of the hollow cylindrical weight CB, which will therefore float in the fluid in such a way, that the quan tity of fluid displaced will be equal in weight to the whole cylinder. When the mouth A of the instrument is exposed to the wind, the weight BC, will descend, and will evidently oppose a resistance to the wind, gra dually increasing till it is submerged in the fluid. The upper surface of the weight, or the surface of the fluid, will indicate on a scale, either fixed or engravers upon the tube, the corresponding force of the wind. By suit ing the density of the fluid and that of the weight to the size of the aperture, the instrument may be made to measure winds of any velocity. There must he air holes near the top of the cylinder, and also in the glass tube, to permit the weight to descend, and the fluid to rise, without resistance from the included air. A hollow tube of glass, closed above and open below, might be immersed in the fluid in place of the weight BC, and the elasticity of the included air would be the resistance opposed to the wind acting upon the upper side of it. It is needless to dwell in tedious detail upon the con struction and graduation of these instruments. It is sufficient, for the direction of the ingenious mechanic, to notice the principles of their construction. A variety of other modes might have been suggested for balanc ing the force of the wind, such as the centrifugal force, as employed in the conical pendulum, the ascent of a weight along an inclined plane, &c. but the limits of our
work will not permit us to prolong the discussion.
We shall now conclude the article with a table for the theoretical graduation of anemometers, which has been completed with no inconsiderable labour. The first column contains the height of a column of water in Dr Lind's anemometer, which corresponds with the force of the wind in column 2d, containing that force in pounds, and decimals of a pound avoirdupois. The third column is the same with the second, excepting that the force is expressed in pounds, ounces, and drams. The fifth and sixth columns shew the velocity of the wind in feet and miles, according to the experiments of Mr Rouse ; and the sixth and seventh contain the velocity according to the more recent and accurate experiments of Dr Hut ton. The eighth column chews the names which dif ferent philosophers have given to winds of a certain In tensity.
For farther information on the subject of anemometers, see Birch's Hist. of the Royal Society. vol. ii. p. 258. IVolfii Opera Mathemat. vol. i. p. 773. Martin's Philo soph. Britann. vol. ii. p. 211. Mem. ?ead. Par. 1734, part. i. p. 169, 8vo. ; p. 124, 4to. Bouguer's Traite du .Va vire,liv. iii. sect. i. chap. ii. p. 359. Bouguer's ilIanatt vres des Vaisseaux, p. 151. Zeiher, .7trov. Comment. P etrop. 1766, vol. x. p. 302. Smeaton's Experimental Enquiry, 8vo. p. 39 ; or Phil. Trans. 1759. vol. li. p. 138. De mengc in Rozier's Observations, vol. xv. p. 433. Poleni, de la Meilleure Maniere de Mesurer sur Ater le C'hemin d'un Vaisseau. Pickering, Phil. Trans. 1744-5, vol. xliii. p. 9. Lomonosow, Azov. Comment. Petrol!. 1749, vol. ii. p. 129. Lind, Phil. Trans. 1775, vol. lxv. p. 353. Le f ons de Physique de L'Ecole Polytechnique, par Pujoulx, p. 201. Bouvet, Recueil des Machines, torn. vi. p. 153. Gustcau, Memoires de Meteorologie, par P. Cottc. Dal berg, in Rozier's Observations, 1781, vol. xvii. p. 438. Leslie's Inquiry into the Nature of Heat, p.284.
Theorie de la Jllanunvre reduite en Pratique, par Pilot.
Phil. Trans. 1766, vol. lv p. 226. Derham, Phil. Trans. 1708-9, vol. xxvi. Manchester Memoirs, vol. iv. Watson's Translat. of Euler on the the Theory of p. 161. Hutton's Translat. of Ozanam's Recreations, vol. i. Young's Nat. Philosophy, vol.. ii. p. 455, 458. (()