Investigations in the U. *S'. Signal Office. 49 



a matter of no small pecuniary importance. These experiments 

 show that, as was formerly lielieved to be the case, the wind 

 pressure varies as the square of the velocity of the wind, expressed 

 in miles per hour ; but a most important fact has developed, 

 namely, that the pressure in pounds per square foot is equal to 

 the miles of hourly velocity multiplied by 0.004 instead of 0.005, 

 as was formerly assumed. 



Professor Marvin was not content with one system of experi- 

 ments, but he further attacked the problem in a direct manner 

 by a method which checked and verified his experiments with 

 the whirling machine. On the summit of Mount Washington, 

 at an elevation of 6,30') feet, he obtained simultaneously and 

 under the same conditions, by automatic and electrical apparatus, 

 continuous registration of the pressure of the wind in pounds 

 per square foot and of the velocity in miles per hour. 



The results thus verified can be considered as conclusive from 

 a general standpoint. The corrections for the Robinson ane- 

 mometer thus determined from these experiments are compara- 

 tively unimportant at Ioav velocities, say from 10 to 15 miles per 

 hour, being only a fraction of a mile per hour. The uncorrected 

 velocities, however, are in all cases too large, and by greater and 

 greater amounts the higher the velocity. At 60 miles per hour 

 the observed velocities are about 12 miles per hour too high, and 

 for an indicated velocity of 90 miles the experiments show that 

 the actual velocity is l:)ut a fraction over 69 miles per hour. 



The anemometer formula found to satisfy most closely the 

 entire range of experiments has the following form for velocities 

 in miles per hour : 



Log. T'= 0.509 + 0.9012 log. r. 



This difference indicated by the formula may seem small and 

 insignificant, as it is in the case of light winds, but at very high 

 velocities the differences are very great. For instance, an actual 

 velocity of 60 miles per hour may occur at some time in almost 

 any locality of the United States for a few minutes, and even 

 greater velocities are occasionally reported, apart from severe 

 tornadoes. Under the old coefficients for the Robinson anemom- 

 eter an actual A^elocity of 60 miles per hour would have been re- 

 ported as 77 miles per hour,, which under the old factor of 0.005 

 would mean a pressure of 29.6 pounds per square foot; but when 

 considered with reference to the true velocity of 60 miles, under 



S— Nat. Geol. Mag., vol. Ill, 1801. 



