54 S. jP. Langley — Internal Work of the Wind. 



But although to show how this physical miracle of nature is 

 to be imitated, completely and in detail, may be found to tran- 

 scend any power of analysis, I hope to show that this may be 

 possible without invoking the asserted power of "aspiration" 

 relative to curved surfaces, or the trend of upward currents, 

 and even to indicate the probability that the mechanical solu- 

 tion of this problem may not be beyond human skill. 



To this conclusion we are invited by the following consid- 

 eration, among others. 



We will presently examine the means of utilizing this poten- 

 tiality of internal work in order to cause an inert body wholly 

 unrestricted in its motion and wholly immersed in the current, 

 to rise / but first let us consider such a body (a plane) whose 

 movement is restricted to a horizontal direction, but which is 

 free to move between frictionless vertical guides. Let it be 

 inclined upward at a small angle towards a horizontal wind so 

 that only the vertical component of the pressure of the wind 

 on the plane will affect its motion. If the velocity of the 

 wind be sufficient, the vertical component of pressure will 

 equal or exceed the weight of the plane, and in the latter case, 

 the plane will rise indefinitely. 



Thus, to take a concrete example, if the plane be a rect- 

 angle whose length is six times its width, having an area of 

 2*3 square feet to the pound, and be inclined at an angle of 7°, 

 and if the wind have a velocity of 36 feet per second, experi- 

 ment shows that the upward pressure will exceed the weight 

 of the plane, and the plane will rise, if between vertical nearly 

 frictionless guides, at an increasing rate, until it has a velocity 

 of 2*52 feet per second,* at which speed the weight and up- 

 ward pressure are in equilibrium. Hence there are no unbal- 

 anced forces acting, and the plane will have attained a state of 

 uniform motion. 



For a wind that blows during 10 seconds, the plane will 

 therefore rise about 25 feet. At the beginning of the motion, 

 the inertia of the plane makes the rate of rise less than the 

 uniform rate, but at the end of 10 seconds, the inertia will cause 

 the plane to ascend a short distance after the wind has ceased, 

 so that the deficit at the beginning will be counterbalanced 

 by the excess at the end of the assigned interval. 



We have just been speaking of a material heavy plane per- 

 manently sustained in vertical guides, which are essential to its 

 continuous ascent in a uniform wind, but such a plane will be 

 lifted and sustained momentarily even if there be no vertical 

 guides, or in the case of a kite, even if there be no cord to 

 retain it, the inertia of the body supplying for a brief period, 



* See "Experiments in Aerodynanrcs," by S. P. Langley, Smithsonian Contri- 

 butions to Knowledge, 1891. 



