SOME NEW BOOKS 



The Internal Work of the Wind. By S. P. Langley. Published by the 

 Smithsonian Institution, Washington, 1893. 



To understand how birds fly, and to imitate them ourselves, 

 were primitive ambitions of the race ; and ambitions, in spite 

 of Mr. Maxim's aeroplanes, they remain. By severe exertion 

 Lilienthal was able, for a few moments at a time, to develop 

 an ascensional force equal to something less than his own weight. 

 The condor and other large birds poise themselves for hours 

 in mid-air, rising, falling, and progressing with no apparent effort, 

 and without the slightest vibratory movement of their expanded 

 wings. The omission of some important factor in the theory can 

 alone account for so great a discrepancy in the result. That this 

 factor is in some way the wind has been long suspected ; but the idea, 

 sometimes held, that a uniform current (however strong) can for long 

 assist the soarer, is at variance with the elementary principles of 

 dynamics. In order that the bird may rise like a paper kite there 

 must needs be something to take the place of the string. In a 

 uniform current there is no force available for the purpose except the 

 bird's inertia during the first few seconds after leaving the ground ; 

 but that birds do use their inertia, at all events in this limited way, is 

 proved by the observation that many of them are unable to soar 

 without facing to windward at starting. If, however, the current is 

 Ho^ uniform, the limitation disappears, for in passing from a quicker to 

 a slower current, or vice versa, inertia again comes into play. The 

 question, therefore, remains, do such inequalities exist, and if so, 

 are they adequate to account for the phenomena ? Professor 

 Langley's paper answers both parts of the question affirmatively, 

 and it is not too much to say that his experiments constitute by 

 much the most important advance yet made in the theory of aerial 

 flight. "In the ordinary use of the anemometer," he writes, "the 

 registry is seldom taken as often as once a minute." His first 

 experiment was to place an ordinary Robinson anemometer on a 

 mast fifty-three feet high and connect it electrically with the chrono- 

 graph of the Allegheny Observatory, so as to register every 25 

 revolutions of the cups. The results are drawn out in a diagram 

 with abscissae = Time, and ordinates = Wind velocity. In a period 

 of about five minutes, taken at random from the diagram, the wind 

 velocity is seen to have fallen from 13 miles an hour to 9, risen to 17, 

 fallen to 14^, risen to 15, fallen to 14, risen to 19, fallen to 13, risen 

 to i8i, fallen to 14, and risen to 17. This degree of variability in 

 *' the wind " is remarkable enough, but Professor Langley's research 

 did not stop here. He proceeded to lighten the cups and to make 

 an automatic record of every revolution. The resulting curve often 

 exhibiting nine or ten cusps within a single minute, and deviations of 

 3c or 40 per cent, on both sides of the line of mean velocity, reveals 



