12 THE INTERNAL WOKE OF THE "WIND. 



by these curves, a veiy brief time before this air reacted the anemometer, and 

 losing it as soon after. 



It Las not been my purpose in this paper to enter upon any inquiry as to the 

 cause of this non-homogeneity of the wind. The irregularities of the surface topog- 

 raphy (including buildings, and every other surface obstruction) are commonly 

 adduced as a sufficient explanation of the chief irregularities of the surface wind ; 

 yet I believe that, a considerable distance above the earth's surface (e. g. one mile), 

 the wind may not even be approximately homogeneous, nor have an even flow ; for 

 while, if we consider air as an absolutely elastic and frictionless fluid, any motion 

 impressed upon it would be preserved forever, and the actual irregularities of the 

 wind would be the results of changes made at any past time, however remote ; so 

 long as we admit that the wind, without being absolutely elastic and frictionless, 

 is nearly so, it seems to me that we may consider that the incessant alterations which 

 it here appears make the " wind," are due to past impulses and changes which are 

 preserved in it, and which die away with very considerable slowness. If this be the 

 case, it is less difficult to see how even in the upper air, and at every altitude, we 

 might e'xpect to find local variations, or pulsations, not unlike those which we 

 certainly observe at minor altitudes above the ground.* 



* In this connection, reference may be made to the notable investigations of Helmholtz, on 

 Atmospheric Movements, Sitzungsberichte, Berlin, 1888-1889. 



