G. I. TAYLOR ON EDDY MOTION IN THE ATMOSPHERE. 



17 



gradient direction is due to the retarding of the wind velocity below the gradient 

 velocity by friction or by viscosity. One might expect, therefore, that the wind 

 would attain the gradient direction at the same height as the gradient velocity. This 

 would, in fact, follow from the theory of GULDBERG and MOHN. Most observations 

 have failed to give reliable information on this point, partly because irregularities on 

 the surface of the earth have introduced complicated conditions, which cannot be 

 taken account of, and partly because the observations have not been grouped according 

 to the wind velocity.* Neither of these objections applies to Mr. DOBSON'S observa- 

 tions. Salisbury Plain, though inferior to the sea, is as good a place for wind 



TABLE II. 



Observed value of 



Qs 



Qo' 



Observed angle a. 



Light winds. 



72 



Moderate winds. Strong winds. 



61 



13 degrees 



a, calculated from (17) so 

 as to correspond with 

 the observed value of 

 Qs 



QG' 



a, calculated from GULD- 

 BERG and MOHN'S 

 theory so as to corre- 

 spond with the observed 



value of -? . 



14 degrees 



2 1|. degrees 



18 degrees 



20 degrees 



20 degrees 



44 decrees 



49 degrees 



52 degrees 



observations near the surface as one could find on land ; and as has been explained 

 already, his results are grouped according to wind velocity. Mr. DOBSON finds that 

 the gradient direction is not attained till a height is reached which is more than twice 

 the height at which the gradient velocity is first attained. He remarks, in fact, that 

 the gradient velocity is usually attained at a height of 300 metres, though the gradient 

 direction is not found till a height of 800 metres has been attained. This is a most 

 remarkable result, but it might have been expected from the equations (12) and (13). 

 The height at which the gradient direction is attained is given by putting v = in 

 (12). If H! be the height in question 



A 2 sin BHi + A 4 cos BE^ = 



* Owing to the fact that p,/p depends on the wind force we should evidently expect more consistent 

 results when the observations are grouped according to wind velocity. 

 VOL. CCXV. A. D 



