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THE ROYAL SOCIETY OF CANADA 



to the length of the arms, that it was less than three, and in some cases 

 varied with the velocity. Dines found that the factor for the Standard 

 Robinson Anemometer at Kew, which has cups 9" in diameter, on 

 arms 2' long, was 2-2 instead of 3. 4" cups on arms 5-8" long gave a 

 factor of 2-51, and V/i' cups on arms 6-75" long gave a factor of 

 2-96, or practically 3. Marvin {loc. cit.) deduced the following 

 equations as more nearly expressing the true value of the wind from 

 the run of the cups for an anemometer with 4" cups on 6-72" arms 

 (the same as the Canadian instrument). 



(1) V= •225 + 3- 143v--0362v2 (whirling machine) 



(2) V= • 263 + 2 •953v- •0407v2 (reduced to open air) 



(3) Log V= -509+ -9012 log V 



where V is the velocity of the wind and v the velocity of thç cup 

 centres. Equation (1) was obtained from observations on a whirling 

 machine in an enclosed space, while (2) and (3) were obtained from 

 whirling machine observations reduced to open air conditions. 



The significance of these factors will be seen from table I, where 

 the wnnd velocities as derived from the different factors are tabulated. 



Table I 



(1), (2) and (3) are Marvin's equations. 



According to the anemometer readings the wind velocity would 

 be given by the factor "3," and the above table shows how much the 

 actual velocity differs from that recorded if these different factors are 

 correct for the instruments to which they refer. Special interest 

 attaches to the velocities given by Marvin's equations as they are for 

 an anemometer of the same proportions as the Canadian. The 

 equations are empirical and were deduced from observations on wind 

 velocities under 35 miles per hour; on this account equations (1) and 

 (2) cannot be used for velocities much over 30 miles per hour, but 

 Marvin states his belief that equation (3) more nearly represents the 

 true velocity as given by the anemometer in use in the United States. 



