436 Prof. 8. P. Langley on the 



any point represents the elapsed time in seconds, and the 

 ordinates show the distance, in feet, of the material particle 

 from the starting-point. The path of the first particle will 

 thus be represented by a straight line, while the path of the 

 second particle will be an irregularly curved line, at one time 

 above, and at another time below the mean straight line just 

 described, but terminating in coincidence with it at the end 

 of the interval. If, now, all the particles in two miles of 

 wind were simultaneously accelerated and retarded in the 

 same way as this second particle, that is, if the wind were an 

 inelastic fluid and moved like a solid cylinder, the velocities 

 recorded by the anemometer would be identical with those 

 that obtained along the whole region specified. But the 

 actual circumstance must evidently be far different from this, 

 since the air is an elastic and nearly perfect fluid, subject to 

 condensation and rarefaction. Hence the successive velocities 

 of any given particle (which are in reality the resultant of 

 incessant changes in all directions) must be conceived as 

 evanescent, taking on something like the sequence recorded 

 by these curves a very brief time before this air reached the 

 anemometer, and losing it as soon after. 



It has 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 topography (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 at a considerable dis- 

 tance 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 for ever, and the actual irregularities of the wind 

 would be the results of changes made at any past time, how- 

 ever 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 alternations, 

 which it here appears make the " wind," are due to past im- 

 pulses 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 expect to find local variations, or 

 pulsations, not unlike those which we certainly observe at 

 minor altitudes above the ground *. 



* In this connexion, reference may be made to the notable investigations 

 of Helmholtz on Atmospheric movements, Sitzungsberichte y Berlin, 1888 



-1889. , 



