38 Mr Forbes's description of a New Anemometer. 



the wind has blown with each velocity since the last observa- 

 tion ; for, unless the dropping of the spherules were performed 

 by clock-work (^and I have thought it more fitted for general 

 adoption to employ the force of the wind merely,) the numbers 

 would be proportioned to the revolutions of the sail A, which, 

 without any danger of vitiating the accuracy of the instrument, 

 which throughout it has been my object to construct on prin- 

 ciples submissible to rigorous calculation, may be considered 

 j)7'oportional to tlie force of the wind, or the square of its velo- 

 city ; for we have nothing to do with the absolute number of 

 rotations occasioned by any given impulsion, — a problem, how- 

 ever, which would meet with constant solutions in the use of 

 the instrument, by comparing the force of the wind computed 

 from the average deflection with the total number of balls 

 fallen during a definite period. The spaces might, however, be 

 so regulated as to indicate the velocities by the number of balls 

 accumulated in them ; but this we shall notice presently, giv- 

 ing, in the first place, the application of the theory of deflection 

 to the instrument. 



Let us assume the spherules to be one-fifth of an inch in 

 diameter, and that they have the density of water, and that the 

 height they fall through is four feet. Since 0.2 inches = ^^ 

 foot, and since a cubic foot of water weighs 62.5 lbs. we have 



62.5 X ^ 

 the weight of a spherule = W = — ^„ — - = .0001515 lb. 



We have seen that by the Newtonian theory R = ^jj\ij of a 



ir72 ^"^ 



^a =z ~. Hence then, 

 S 



Logarithm R, . . 4.50111 



rf2 __ .04 inches, log. . . 8.60206 



Resistance to vel. 1. . . 3.10317 



Log. W. . . 6.18041 



Diff. = log. m2 . . 3.07724 



g= 32 log. . . 1.50515 



2a = 37. 33 feet. log. . . 1.57209 



Then, V being the initial velocity, or the velocity of the wind 



in feet per second, e = the extinguishing time = -y : And if 



