62 THE MIGRATION OF BIRDS 



were guided by the beam-winds ; always keeping 

 the wind on their flanks led them aright (14). He 

 says that if they fly at 100 miles per hour, with a 

 beam-wind of 30 miles per hour, they will reach a 

 spot 100 miles from whence they started, but 30 

 miles to leeward of a line drawn at right angles to 

 the wind. Thus, if they rely upon the wind, their 

 course is more or less diagonally across it according 

 to its strength. He maps out the supposed route 

 according to prevailing winds, but fails to notice 

 that the very route he maps may be caused simply 

 by the leeward drift when flying on winds which 

 are not with them. One portion of the journey is 

 enough to illustrate what I mean. From Labrador 

 to the east of Bermuda the birds fly south-east, so, 

 he argues, as to cross the south-west wind at right 

 angles. But supposing the birds headed due south, 

 meeting the south-west 'wind on their right front, 

 they would of necessity, if the wind was strong, 

 drift away to the east. It is improbable that they 

 actually aim to strike the Bermudas, for it is only 

 during certain weather conditions that they visit 

 these islands. In favourable weather the birds do 

 not touch the Bermudas, but continue their flight 

 direct to South America. 



The leeward drift of birds in a strong beam-wind 

 may be noticed during ordinary flight ; it has 

 occasioned one of the most remarkable of Gatke's 



