Internal Work of the Wind. 143 



tamable at any height, gravity may be utilized to cause the 

 body (which we suppose to be a material plane) to descend 

 on an inclined course, to some distance, even against the 

 wind. 



I desire, in this connexion, to remark that the preceding 

 experiments and deductions, showing that a material free 

 plane "*, possessing sufficient inertia, may in theory rise in- 

 definitely by the action of an ordinary wind, without the 

 expenditure of work from any internal source (as well as 

 those statements which follow), when these explanations are 

 once made, have a character of obviousness, which is due to 

 the simplicity of the enunciation, but not, I think, to the 

 familiarity of the explanation, for though attention is be- 

 ginning to be paid by meteorologists to the rapidity of these 

 wind fluctuations, I am not aware that their effects have 

 been so exhibited, or, especially, that they have been pre- 

 sented in this connexion, or that the conclusions which follow 

 have been drawn from them. 



We have here seen, then, how pulsations of sufficient 

 amplitude and frequency, of the kind which present them- 

 selves in nature, may, in theory, furnish energy not only 

 sufficient to sustain, but actually to elevate, a heavy body 

 moving in and with the wind at its mean rate. 



It is easy now to pass to the practical case which has been 

 already referred to, and which is exemplified in nature, 

 namely, that in which the body (e.g. the bird soaring on rigid 

 wings, but having power to change its inclination) uses the 

 elevation thus gained to move against the wind, without 

 expending any sensible amount of its own energy. Here 

 the upward motion is designedly arrested at any convenient 

 stage, e. g. at each alternate pulsation of the wind, and the 

 height attained is utilized so that the action of gravity may 

 carry the body by its descent in a curvilinear path (if 

 necessary) against the wind. It has just been pointed out 

 that if some height has been attained, the theoretical possi- 

 bility of some advance against the wind in so falling hardly 

 needs demonstration, though it may not unnaturally be sup- 

 posed that the relative advance so gained must be insignificant, 

 compared with the distance travelled by the mean wind while 



* I use the word " plane," but include in the statement all suitable 

 modifications of a curved surface. 



I desire to recall attention to the paragraph in tf Experiments in Aero- 

 dynamics," in which I caution the reader against supposing that by- 

 investigating plane surfaces I imply that they are the best form of sur- 

 face for flight ; and I repeat here that, as a matter of fact, I do not 

 believe them to be so. I have selected the plane simply as the best form 

 for preliminary experiment. 



2H2 



