56 



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



are in opposite directions, and will produce on a plane, whose 

 inertia enables it to sustain a sensibly uniform motion with the 

 mean velocity of this variable wind, the same lifting effect as 

 if these same alternating winds were in absolutely opposed 

 directions, provided that the (constant inclination of the plane 

 alternates in its aspect to correspond with the changes in the 

 wind. 



It may aid in clearness of concep- 

 tion, if we imagine a set of fixed co- 

 ordinates X Y Z passing through 0, 

 and a set of movable co-ordinates 

 x y s, moving with the velocity and 

 the direction of the mean wind. If 

 the moving body is referred to these 

 first only, it is evidently subject to 

 pulsations which take place. in the 

 same directions on the axis of X, but 

 it must be also evident that if re- 

 ferred to the second or movable co- 

 ordinates, these same pulsations may 

 be and are, in opposite directions. 

 This, then, is the case we have just 

 considered, and if we suppose the 

 plane to change the aspect* of its 

 (constant) inclination as the direc- 

 tion of the pulsations changes, it is 

 evident that there must be a gain in 

 altitude with every pulsation, while 

 the plane advances horizontally with the velocity of the mean 

 wind. 



During the period of maximum wind velocity, when the 

 wind is moving faster than the plane, the rear edge of the lat- 

 ter must elevated. During the period of minimum velocity, 

 when the plane, owing to its inertia is moving faster than the 

 wind, the front edge of the plane must be elevated. Thus the 

 vertical component of the wind pressure as it strikes the 

 oblique plane, tends in both cases, to give it a vertical upward 

 thrust. So long as this thrust is in excess of the weight to be 

 lifted, the plane will rise. The rate of rise will be greatest at 

 the beginning of each period, when the relative velocity is 

 greatest, and will diminish as the resistance produces " drift ;" 

 i. e. diminishes relative velocity. The curved line O B in the 

 vignette, represents a typical path of the plane under these 

 conditions. 



* We do not for the moment consider how this change of aspect is to be mechani- 

 cally effected; we only at present call attention to the fact that it involves, in 

 theory, no expenditures of energy. 



Tig. 2. 



