THE INTERNAL WoKK OF THE WIND. 19 



deductions, showing that ;i material free plane,* possessing sufficient inertia, may in 

 theory rise indefinitely 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 explana- 

 tion ; for though attention is beginning 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 presented in this connection, or that the conclu- 

 sions which follow have been drawn from them. 



We have here seen, then, how pulsations of sufficient amplitude and frequency, 

 of the kind which present themselves 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 to now pass to the practical case which has been already referred to, 

 and which is exemplified in nature ; namely, that in which the body (<?. g. the bird 

 soaring on rigid wings, but having power to change its inclination) uses the eleva- 

 tion 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 conven- 

 ient stage, e. g. at each alternate pulsation of the wind, and the height attained is 

 utilized so that the reaction of gravity may carry the body by its descent in a curvi- 

 linear path (if necessary) against the wind. It has just been pointed out that if 

 some height has been attained, the theoretical possibility of some advance against 

 the wind in so falling hardly needs demonstration, though it may not unnaturally 

 be supposed that the relative advance so gained must be insignificant, compared 

 with the distance travelled by the mean wind while the body was being elevated, 

 so that on the whole the body is carried by the wind farther than it advances 

 against it. 



This, however, probably need not be in fact the case, there being, as it appears 

 to me, from experiment and from deduction, every reason to believe that under 

 suitable conditions, the advance may be greater than the recession, or that the body, 

 falling under the action of gravity along a suitable path, may return against the 

 wind not only from Z to O, the point of departure, but farther, as is here shown. 



I repeat, however, that I am not at the moment undertaking to demonstrate 



* 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 Experiments in Aerodynamics in which I 

 caution the reader against supposing that by investigating plane surfaces I imply that they are the 

 best form of surface for flight ; and I repeat here that, as a matte] oJ fa< t, I do not believe them to 

 be so. I have selected the plane simply as the best form for preliminary experiment. 



