270 FLIGHT 



the wind, while in going up into the wind it rises higher in the air. 

 The bird may, in describing these curves or circles, rise as high or 

 higher than the point from which it started, and may be as far or 

 further to windward, and this without any very evident expenditure 

 of mechanical work on the part of the bird. This at first sight looks 

 sufficiently startling, and one's first impulse would naturally be to 

 question the facts. There appears, hoAvever, to be no sufficient 

 reason for doubting the main points above stated. Other modes of 

 soai'ing have, indeed, been described, but it is unnecessary to go 

 into these latter, since any explanation of how it is that a bird, with 

 what may be described as little or no mechanical work ^ on its part, 

 can not only keep itself up in the air but can actually rise higher, 

 or, keeping about the same level, can progress to windward, Anil 

 presumably cover all the varieties of soaring. 



Now the theories with which we are acquainted as to the 

 mechanics of soaring may be divided into tAvo categories — those, 

 in the first place, evolved by observers Avho have noted certain 

 prominent facts and have sought to explain these by reasonings 

 Avhich have been in some cases Avildly disregardful of the elements 

 of dynamics. Such theories may, indeed must, be put to one side, 

 although the facts on which they are based cannot be left out of 

 consideration. The second category of theories are those made by 

 physicists and mathematicians, and Avhich are characterized by 

 being in harmony Avith what is known of the laws of nature, and 

 also, Avith certain important exceptions (to be presently considered), 

 by being inadequate to explain facts which had been noted by a 

 consensus of trustworthy observers. We cannot go over these tAvo 

 series of theories in detail, and can only find room to consider two, 

 drawn from our second category. These are, first, the theory of 

 upward currents of air ; ^ and, secondly, the theory of varying velocity of 

 the wind at different heights from the land or water. There is a good 

 deal to be said for both of these possible explanations — Avhich, 

 of course, involves the conclusion that the data required to decide 

 the matter are still wanting. Nevertheless, the subject is too 

 interesting and important to justify us in seeking to eA^ade the 

 difficulties of the problem before us, and we aa^II try to put seriatim 

 the reason for and against these two (so far as Ave knoAV only) 

 possibilities. 



1 While resting on its motionless extended wings a bird may be doing no 

 mechanical work, but it is nevertheless expending energy in keeping up a certain 

 degree of contraction of its pectoral muscles. This work, which is real enough of 

 its kind, is expended in the muscles themselves, and concerns the physiologist 

 rather than the mechanician. It is called internal work by physiologists. . 



- W. Froude, Proc. R. Soc. Edinh. xv. pp. 256-258 (19 March 1888). [A con- 

 tinuation of this note has since appeared {op. cit. xviii. pp. 65-72, 5 January 

 1891).— A. N.] 



