AIR CURRENTS IN RELATION TO AVIATION 347 



I shall call attention to the want of homogeneity of air 

 currents, to the manner in which a broad current seems to be 

 formed by streams of different kinds of air flowing alongside, 

 out of which there appear to be developed, under suitable condi- 

 tions, the phenomena of the line-squall. These are the bores 

 and perhaps the whirlpools of the atmosphere. 



I propose to say a good deal about the line-squall, because it 

 is characteristic of the most vindictive and most destructive 

 phenomena of weather. It would appear that in Greek mytho- 

 logy the line-squall is personified in the Harpies, "the 

 Snatchers." I shall have to describe some of the characteristic 

 phenomena of the line-squall before I can make good this sug- 

 gestion but I notice that there is a tendency in these days to 

 revert to Greek mythology for appropriate meteorological ex- 

 pression. If " anticyclone " is to be displaced by the more poetic 

 name " halcyon," that has its associations with the kingfisher, 

 let me suggest that the airman should learn to look upon a line- 

 squall as a harpy and deal with it accordingly. 



Under the second heading, " The Texture of Air Currents," 

 I shall consider the variations in a homogeneous current and 

 show that in the atmosphere what passes for a steady blow is 

 not by any means free from variations in direction and speed. 

 I shall show what we have been able to make out of the details 

 of these variations. They are analogues of the waves of the sea 

 as regards the comparative rapidity of the variations but we shall 

 see that they cannot be called waves in any dynamical sense. 



Structure 



In dealing with the structure of the atmosphere I shall take 

 first the phenomena of the layer within which the friction of the 

 surface is still operative and the relation of that layer to the 

 upper undisturbed currents. In dealing with this part of our 

 subject, when we consider the changes which take place, we 

 require a standard to which the variations upon any occasion 

 can be referred. One might naturally suppose that the surface 

 wind would be the proper standard ; for the purposes which 

 I have in view, it is not at all a convenient standard because the 

 measure of the surface wind depends so largely upon the ex- 

 posure of the particular anemometer which measures it. The 

 surface wind at two places not many miles or even many yards 



