21G REPORT ON THE SURFACE DRIFT OF THE ENGLISH CHANNEL 



with the other figures based only upon two observations daily, we get 

 the mean of these three resultants as follows : — 



,, 38 + 15-5 + G-3 59-8 .^n r n on 



Mean = = — — - = 19-9, or, practically 20, 



6 6 



which is the measure in pounds per foot of the resultant of the wind- 

 pressures, taken twice daily, during the period of drift. 



The mean average pressure is, of course, this sum divided by the num- 



ber of days and the number of observations per diem, or -^ — - = 3"3, 



O X ^ 



which is equivalent to a velocity of 26 miles per hour, or a " force " 

 intermediate between 4 and 5 (4"6) on the Beaufort scale. 



The resultant or minimum distance traversed by the drift-bottle from 

 the Eddystone to Portscatho = 25 geographical miles. 



We conclude therefore that a wind exerting a horizontal pressure of 

 3'3 pounds per foot ( = a velocity of 26 miles per hour, or a force inter- 

 mediate between 4 and 5), and blowing steadily for three days, will 

 cause a surface drift in the same direction of 25 miles in that time, i.e., 

 a current having a surface velocity of 8*3 miles per diem. 



This result is considerably lower than that attained by Mohn, and 

 bears out my remarks concerning the inapplicability of Mohn's current- 

 equivalent to cases of currents produced by the wind from rest. 



Having thus determined the drift-equivalent, it is easy to construct a 

 factor from it by which the resultant wind pressure for any period may 

 be quickly converted into the number of miles travelled by the surface 

 drift for the same period. 



If D be the Distance travelled in miles, and Po the resultant Wind- 

 Pressure in pounds per foot computed from 2 observations daily, 



then D : Po : : 25 : 20, 

 orD = ^- 



2 

 and since Po may be taken as merely - (P„), where P,i is the Eesultant 



Pressure determined from n observations daily, we have the general 

 formula 



^•) 



^ ^ 5_P2 ^ Vn'V ^ 5 X 2 P„ _ 5 P, 



4 4: X n 2n 



In most cases it is, of course, sufficient to determine the resultant wind 

 and wind pressure for any period from the observations recorded twice 



