58 S. P. Langley — Internal Work of the Wind. 



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 connection to remark that the preceding 

 experiments and deductions showing that a material free 

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

 nitely by the action of an ordinary wind, without the expendi- 

 ture of work from any internal source (as well as those state- 

 ments, 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 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 espe- 

 cially, that they have been presented in this connection, or 

 that the conclusions which follow have been drawn from them. 



We have here seen, then, how pulsations of sufficient ampli- 

 tude 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 (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 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 traveled by the mean wind while the body was being 

 elevated, so that on the whole the body is carried by the wind, 

 further 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, 



*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 sur- 

 faces, I imply that they are the best form of surface 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. 



