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, however, to be no sufficient 
reason for doubting the main points above stated. Other modes of 
soaring 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, will 
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 two categories—those, 
in the first place, evolved by observers who have noted certain 
prominent facts and have sought to explain these by reasonings 
which have been in some cases wildly 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 which are characterized by 
being in harmony with what is known of the laws of nature, and 
also, with 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 two 
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—which, 
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 evade the 
difficulties of the problem before us, and we will try to put seriatim 
the reason for and against these two (so far as we know 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. 
2 W. Froude, Proc. R. Soc. Edinb. 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. Nw ; 
