NO. 8 SA^[UEI- PIERPONT LANGLEY ABBOT 4I 



much higher than those he tried. His assumption that skin friction is 

 negHgible is also invahd at higher speeds. Rut a great impetus to 

 aviation was given by the fact that so great a scientist as Langley had 

 devoted himself to a subject which was generally regarded then as 

 the refuge of cranks, nearly in the same class with perpetual motion. 



Langley's meditations on soaring flight of birds led in 1893 to his 

 brilliant paper : 



"THE INTERNAL WORK OF THE WIND" 



" It has long been observed that certain species of birds maintain 

 themselves indefinitely in the air by ' soaring,' without any flapping 

 of the wing, or any motion other than a slight rocking of the body; 

 and this, although the body in question is many hundred times denser 

 than the air in which it seems to float with an undulating movement, 

 as on the waves of an invisible stream. 



" No satisfactory mechanical explanation of this anomaly has been 

 given, and none would be offered in this connection by the writer, 

 were he not satisfied that it involves much more than an ornithological 

 problem, and that it points to novel conclusions of mechanical and 

 utilitarian importance. They are paradoxical at first sight, since they 

 imply that, under certain specified conditions, very heavy bodies en- 

 tirely detached from the earth, immersed in, and free to move in, the 

 air, can be sustained there indefinitely, without any expenditure of 

 energy from within. 



" These bodies may be entirely of mechanical construction, 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 ob- 

 serve 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 observation, that the soaring vultures 

 and other birds appear as if sustained by some invisible support, in 

 the stream of air, sometimes 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 obser- 

 vation, are aware that it is absolutely certain that nothing of the kind 



