Internal Work of the Wind. 439 



We will presently examine the means of utilizing this po- 

 tentiality of internal work in order to cause an inert body 

 wholly unrestricted in its motion and wholly immersed in the 

 current, to vise ; but first let us consider such a body (a plane) 

 whose movement is restricted in 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, 

 experiment shows that the upward pressure will exceed the 

 weight of the plane, and the plane will rise, if between ver- 

 tical nearly frictionless guides, at an increasing rate, until it 

 has a velocity of 2' 52 feet per second *, at which speed the 

 weight and upward pressure are in equilibrium. Hence 

 there are no unbalanced 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 counter- 

 balanced 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 the office of the guides or of the cord. If suit- 

 ably disposed, it will, as the writer has elsewhere shown, 

 under the resistance to the horizontal wind, imposed only by 

 its inertia, commence to move, not in the direction of the 

 wind, but nearly vertically, Presently however, as we recog- 

 nize, this inertia must be overcome, and as the inclined plane 

 takes up more and more the motion of the wind, the lifting- 

 effect must grow less and less (that is to say, if the wind 



* See " Experiments in Aerodynamics," by S. P. Langley, 'Smithsonian 

 Contribution to Knowledge,' 1891. 



