INTRODUCTION. xiii 



of the bird. In flight alone there are problems to 

 keep investigators at work for many yeai*s to come, and 

 there is a further incentive in the likelihood that a 

 more penetrating study of bird-flight would result in 

 practical hints concerning what is somewhat grotesquely 

 called ' human aviation.' Speaking as a general biologist, 

 I confess to finding a peculiar fascination in that mode 

 of flight which is called ' sailing ' or ' soaring,' to which 

 reference is made in the pages that follow. The fascina- 

 tion is partly due to the fact that this kind of flight is 

 still to some extent an unsolved problem ; it is also due 

 to the impression that here one is face to face with the 

 greatest locomotor triumph that life has achieved. 



If we take a readily accessible and authoritative account 

 of the facts, such as Professor C. S. Roy's article on 

 Flight in Newton's Dictionary of Birds, we find that 

 sailing is mostly exhibited by big birds, whose sail-area 

 is large in proportion to the total weight — e.g. Raven, 

 Pelican, Stork, and Albatross ; that a certain amount of 

 wind appears to be essential ; and that the sailing-bird, 

 ' with motionless outstretched wings, describes in its 

 flight curves or circles which lead it to sail alternately 

 up the wind and down the wind.' 



There may be ' internal work ' in keeping the sails taut ; 

 but as there are no active wing-strokes for long periods 

 (one expert observer said not for three-quarters of an 

 hour !), the problem is how the heavy bird can keep itself 

 aloft ; can progress close in the eye of the wind ; can rise 

 and fall and rise again ; can turn on its course and cui've 

 back upon it again, as the albatross certainly does around 

 the ship. Of course, the problem has been often tackled by 

 physicists and ornithologists, and various theories have been 

 suggested — for instance, the thcoiy that the sailing-bird 

 utilises upward currents of air, such as we see illustrated 



