426 Prof. S. P. Langley on the 



as will be seen later, but for the present we will continue to 

 consider the character of the invisible support of the soaring 

 bird, and to study its motions, though only as a pregnant 

 instance offered by nature to show that a rational solution of 

 the mechanical problem is possible. 



Recurring, then, to the illustration just referred to, we may 

 observe that the flow of an ordinary river would afford no 

 explanation of the fact that nearly inert creatures, while free 

 to move, although greatly denser than the fluid, yet float upon 

 it ; which is what we actually behold in the aerial stream, since 

 the writer, like others, has satisfied himself by repeated obser- 

 vation that the soaring vultures and other birds appear as if 

 sustained by some invisible support, in the stream of air, some- 

 times for at least a considerable fraction of an hour. It is 

 frequently suggested by those who know these facts only from 

 books, that there must be some quivering of the wings, so 

 rapid as to escape observation. Those who do know them 

 from observation are aware that it is absolutely certain that 

 nothing of the kind takes place, and that the birds sustain them- 

 selves on pinions which are quite rigid and motionless, except 

 for a rocking or balancing movement involving little energy. 



The writer desires to acknowledge his indebtedness to that 

 most conscientious observer, M. Mouillard *, who has described 

 these actions of the soaring birds with incomparable vividness 

 and minuteness, and who asserts , that they, under certain 

 circumstances, without flapping their wings, rise and actually 

 advance against the wind. 



To the writer, who has himself been attracted from his 

 earliest years to the mystery which has surrounded this action 

 of the soaring bird, it has been a subject of continual surprise 

 that it has attracted so little attention from physicists. That 

 nearly inert bodies, weighing from 5 to 10, and even more, 

 pounds, and many hundred times denser than the air, should 

 be visibly suspended in it above our heads, sometimes for 

 hours at a time, and without falling, — this, it might seem, is. 

 without misuse of language, to be called a physical miracle ; 

 and yet the fact that those whose province it is to investigate 

 nature have hitherto seldom thought it deserving attention 

 is perhaps the greater wonder. 



This indifference may be in some measure explained by the 

 fact that the largest and best soarers are of the vulture kind, 

 and that their most striking evolutions are not to be seen in 

 those regions of the Northern Temperate Zone where the 

 majority of those whose training fits them to study the sub- 



' * L. P. Mouillard, E Empire da VAir, Paris : G. Masson. 



