Mr Forbes's description of a New Anemometer. 39 



we suppose V = 40, which corresponds, according to the expe- 

 riments of Hiitton, to a " very brisk wind,'' e = 0."933S. 

 Since the space fallen through is four feet, t =z time of expo- 



y2 s /8 



o 



rithimic modulus being = 0.43429, &c. we have all the data 

 for solving the equation. 



2a ^ e^t _ ^^ 37.33 ,1.4333 



Log. 2 a . 1.57209 

 M . "9-63778 



Log. e = 9.97004 1.93431 



].e-\- t=0A5635 



1. 1±J 0.18631 « - 2d log. 9.27023 



16.016 log. 1.20454 



Finally ; V / =: 40 x 0.5 = 20.000 feet 

 2« , e + t 

 — M ^ ^''^' e =i^:^ 

 Amount of deflection, 3.984 feet. 

 The case we have just supposed is perhaps one of the most 

 convenient in practice ; * it is besides curious, as showing the 

 theoretical effect of wind on falling drops of rain, which are 

 sometimes of the size we have assumed, and the density is 

 equal. As the deflection, however, is greater than would 

 ever be required for accuracy of measurement, the height of 

 the fall, and consequently the time of exposure, might be re- 

 duced, which would put the instrument within smaller com- 

 pass. Suppose the time halved, or = 0."25, we have s = 1 

 foot, and the last solution is easily altered to the present case. 



Log. e = 9-97004 

 € + ^ = 1.1833 log. 0.07309 



log. -i-' = 0.10305 M log. 9-01305 



log.^ 1.93431 



8.858 log. O.9473Q 

 V/ = 40 X 0.25 = 10.000 feet. 



Deflection — 1 . 1 42 feet. 



* The spherules might be turned out of wood. Perhaps eyen dry peas 

 might answer the purpose. 



