90 THE MIGRATION OF BIRDS 



eddies upon a swift stream/' in the North Atlantic 

 as a rule from south-west to north-east. 



Do we really know the force and direction of the 

 winds at a high altitude during these movements ? 

 Are we not merely guessing at the real aerial con- 

 ditions by the movements near the earth at the 

 time of the departure of the birds ? Is it fair, if 

 I am right that the meteorological observations 

 are founded upon only those observed at com- 

 paratively low altitudes, to lay down laws as to 

 the particular conditions which are favourable or 

 unfavourable, or the particular winds which are 

 used or avoided ? The direction of the wind may 

 be the same up to a great height, many thousand 

 feet, or it may vary within 500 feet of the earth. 

 Nearly fifty years ago, when Glaisher made his 

 great ascents, he sometimes met with three or four 

 currents moving in opposite directions. The more 

 recent upper air investigations show that though 

 as a rule the wind at various elevations is in the 

 main from one point of the compass, its degrees 

 vary considerably, and its force at the various 

 heights shows remarkable differences. Generally 

 the force rises to about 5000 feet, but there is no 

 invariable rule. I tabulate a few examples taken 

 more or less at random from the Weather Reports 

 for 1908. The altitudes above the ground are 

 measured in metres, roughly converted into feet ; 



