558 NEW METHOD OF DETERMINING THE WIND'S VELOCITY 



hand. I do not know that it has ever before been ajoplied in this way, but 

 I find the results obtained agree very closely with those derived from a 

 formula computed by Colonel James for the British Board ot Trade. The 

 formula is this : 



P=z — Fy, 



2a- 



in which P is the pressure of the wind, z a term dependent on the shape ot 



v^ 

 the body exposed to the wind, — the height due to the velocitj^ v, F the 



2g _ 

 exposed area, y the density of the air. The value of z has been found from 

 experiment to be about 0,64, so transposing the equation to find the value 



of V" we have, 



2gP 



V^=: , 



.64 Fy 

 from which the numerical value of v can be easily found, as those of g, P 

 F and z are known. It must be observed that the value of z varies with 

 the barometric height, making it necessary to read the barometer for each 

 set of observations. It is now easy to construct another table, as follows: 



For deflection of 1 cen., velocity = 3.9 m. per h. 



u (I 9 ■' " 5 51 " " 



u u g « « q'j^ i, u 



U II A (< (( y QQ it il 



" " 60 " " 30 2 " " 



Finally these values are substituted in the columns of deflections ob- 

 served, and a mean obtained which represents the wind's velocity for the 

 fifteen minutes of observation. 



After making a large number of experiments as above described, I 

 learned these facts regarding the anemometer at the Kansas University : 

 First, that it never registers too much, and second, that for small velocities 

 it does not register enough. This is due to the fact that a gentle wind, 

 whose strength, however, is sufficient to deflect the ball, will fail to move 

 the cups sometimes for many minutes. 



The following figures will show results obtained by both methods. The 

 first column contains the velocities determined by the deflected ball, and 

 the second those determined for the same time by the anemometer : 



It will be observed that for medium and high velocities the two methods 

 compared very well, and for low velocities, as intimated, the anemometer 

 fails to give a large enough record. Besides showing that the apparatus 

 at Lawrence cannot register too much, my observations have convinced 



