WIND. 



altitude a body defcends in the firft fecond of time, to the 

 aforefaid fpace of the air. . , , . 



Suppofe, <f. ?r. the fpace through which the air moves 

 in a fecond, « = 24 feet, or 288 inches ; call the alti- 

 tude of the fluid x; and the ratio of mercury to air, 



£^8oox_i4^ n2oo_ ^^^ ^^^ ^^^^^^^^ through which 



r I • ' 



a body defcends in the firft fecond of time, 16 feet i 

 inch; then, by the theorem, we (haU have 288 : x :: 



288 X 288 



1 1 200 X 762 : 288, and confequently x = ^^^^ ^ ^^^ 



= 01 &c. of an inch. Hence we fee why a fmall 

 butfudden change in the barometer is foUowed with 

 violent winds. See an account of the principle upon which 

 thefc calculations are founded under the article Water. 



When the direftion of the wind is not perpendicular, but 

 oblique to the furface of the folid, then the force of the 

 former upon the latter will not be fo great as when the im- 

 pulfe is direft, and that for reafons which are eafily derived 

 from the theory of the refolution and compofition of forces, 

 and from the theory of dired and obhque impulfes. In 

 (hort, the general propofition for compound impulfes is, that 

 the effeaive impulfe is as the furface, as the fquare of the 

 air's velocity, as the fquare of the fine of the angle of inci- 

 dence, and as the fine of the obliquity of the foUd's motion 

 to the direaion of the impulfe, jointly ; for the alteration of 

 every one of thofe quantities will alter the effea in the fame 

 proportion. But th.-fe general rules, as we have already 

 more than once obferved, »re fubjea to great variations ; fo 

 that their refults feldom coincide with thofe of aaual 

 experiments. . . 



Philofophers have ufed various methods for determining 

 the velocity of the wind, which is very different at different 

 times. The method ufed by Dr. Derham was that of 

 letting light downy feathers fly in the wind, and accurately 

 obferving the diftance to which they were carried in any 

 number of half feconds. This method he preferred to that 

 of Dr. Hooke's mola alata.or pneumatica. ( See Phil. Tranf. 

 N° 24. and Birch's Hift. Roy. Soc. vol. iv. p. 225.) 

 He tells us, that he thus meafured the velocity of the wind 

 in the great ftorm of Auguft, 1705, and by many experi- 

 ments found, that it moved at the rate of thirty-three feet 

 fer half-fecond, or of forty-five miles per hour : whence he 

 concludes, that the moft vehement wind (as that of Novem- 

 ber, 1703) does not fly at the rate of above fifty or fixty 

 miles per hour, and that at a medium the velocity of wind is 

 at the rate of twelve or fifteen miles per hour. Phil. Tranf. 

 N° 313. or Abr. vol. iv. p. 411. 



Mr. Brice obferves, that experiments with feathers are 

 fubjea to uncertainty : as they feldom or ever defcribe a 

 ftraight line, but defcribe a fort of fpirals, moving to the 

 right and left, and rifing to very different altitudes in their 

 progrefs. He, therefore, confiders the motion of a cloud, 

 or its (hadow, over the furface of the earth, as a much more 

 accurate meafure of the velocity of the wind. In this way 

 he found, that the wind, in a confiderable ftorm, moved at 

 the rate of 62.9 miles per hour ; and that, when it blew a 

 frefti gale, it moved in the fame time about twenty-one 

 miles ; and that id a fmall breeze, the wind moved at the 

 rate of 9.9 miles per hour. Phil. Tranf. vol. Ivi. p. 226. 



But it has been obferved by Cavallo and others, that this 

 method is very fallacious, partly becaufe it is not known 

 whether the clouds do or do not move exaftly with the air 

 in which they float ; and partlv becaufe the velocity of the 

 air in the region where the clouds float is by no means the 



fame with that of the air which is nearer to the furface of 

 the earth, and is fometimes quite contrary to it, as indicated 

 by the motion of the clouds themfelves. Others have efti- 

 mated the velocity of the wind by the changes effeded by 

 it upon the motion of found, which muft of courfe be very 

 inaccurate. A very fimple method of determining the ve- 

 locity of the wind is that which M. Coulomb (Mem. de 

 I'Acad. Roy. 178 1, p. 70.) employed in his experiments 

 on wind-mills, becaufe it requires neither the aid of inftru- 

 ments nor the trouble of calculation. Two perfong were 

 placed on a fmall elevation, at the diftance of 150 feet from 

 one another, in the direaion of the wind ; and, while the 

 one obferved, the other meafured the time which a fmall and 

 light feather employed in removing through this fpace. 

 The diftance between the two perfons, divided by the num- 

 ber of feconds, gave the velocity of the wind per fecond. 

 The beft method, fays Cavallo, of meafuring the velocity 

 of the wind, is by obferving the velocity of the fmoke of a 

 low chimney, or by eftimating the effeft it produces upon 

 certain bodies, and thus may be determined its force as well as 

 its velocity. We fhall here obferve, that from the concurrence 

 of experiments made with various inftruments, and different 

 modes of calculation, it has been inferred, that in currents of 

 air, of the denomination which are expreffed in the 4th 

 column of the annexed table, the air moves at the rate of 

 fo many feet per fecond as are expreffed in the 2d column, 

 or of fo many miles per hour as are expreffed in the ift 

 column. 



A Table of the different velocities and forces of the 

 vnnds, conftruaed by Mr. Roufe with great care, from a 

 confiderable number of faas and experiments, and com- 

 municated to Mr. Smeaton, and firft publilhed by him in 

 the 51ft volume of the Philofophical Tranfaaions. 



The force of the wind is as the fquare of its velocity ; as 

 Mr. Fergufon has fhewn by experiments on the whirling, 

 table i and in moderate velocities this will hold very nearly. 



Upon 



