444 



Prof. S. P. Langley on the 



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 0, the point of 

 departure, but farther, 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 

 purpose, 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 useful 

 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 illustration (fig. 3) we suppose the body to use 

 the height gained in each pulsation, to enable it to descend 

 after each such pulsation and advance against the direction 

 of the wind. 



Tig.3. 



The portion AB 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 



