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



59 



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 further as is here shown. 



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

 to demonstrate how the action is mechanically realizable in 

 actual practice, but only that it is possible. It is for this pur- 

 pose, and to understand more exactly that it can be effected, 

 not only by the process indicated, in the second illustration 

 (fig. 2) but by another and probably more usual one (and nature 

 has still others at command), that I have considered another 

 treatment of the same 'conditions, of wind-pulsations always 

 moving in the same horizontal direction, but for brief periods, 

 interrupted by equal intervals of calm. In this third illustra- 

 tion (tig. 3) we suppose the body to use the height gained in 

 each pulsation, to enable it to descend after each such pulsa- 

 tion and advance against the direction of the wind. 



Fiff.S. 



The portion A B of the curve represents the path of the 

 plane surface from a state of rest at A, where it has a small 

 upward inclination toward the wind. If a horizontal wind 

 blow upon it in the direction of the arrow, the first movement 

 of the plane will not be in the direction of the wind, but as is 

 abundantly demonstrated by the writer in "Experiments in 

 Aerodynamics" it will rise in a nearly vertical direction, if the 

 angle be small. The wind, continuing to blow in the same 

 direction, at the end of a certain time, the plane, which has 

 risen (owing to its inertia, and in spite of its weight), to the 

 successive positions shown, is taking up more and more of the 

 horizontal velocity of the wind, and consequently opposing 

 less resistance to it, and therefore moving more and more .late- 

 rally, and rising less and less, at every successive instant. 



If the wind continued indefinitely, the plane would ulti- 

 mately take up its velocity, and finally, of course, fall, when 

 this inertia ceased to oppose resistance to the wind's advance. 



