THE [NTERNAL WORK OF THE WIND. 15 



whose movement is restricted in ;t horizontal direction, but which is free to rise be- 

 tween Motionless vertical guides. Let it, be inclined upward at a small angle 

 toward u 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 tin- plane will rise indefinitely. 



Thus, to take a concrete example, if the plane be a rectangle whose length 

 is six times its width, having an area of 2.3 square feet to the pound, and lie 

 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 vertical nearly frictionless guides, at an in- 

 creasing 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 counterbalanced by the excess at the end of the assigned interval. 



We have just been speaking of a material heavy plane permanently 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 

 suitably disposed, it will, as the writer has elsewhere shown, under the resistance to 

 a horizontal wind, imposed only by its inertia, commence to move, not in the direc- 

 tion of the wind, but nearly vertically. Presently, however, as we recognize, 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 be the approximately homogeneous current it is commonly treated as being), 

 and finally ceasing altogether, the plane must ultimately fall. If, however, a counter- 

 current is supposed to meet this inclined plane, before the effect of its inertia is 

 exhausted, and consequently before it ceases to rise, Ave have onl}" to suppose the 

 plane to be rotated through 180° about a vertical axis, without any other call for 

 the expenditure of energy, to see that it will now be lifted still higher, owing to the 



* See Experiments in Aerodynamics, by S. P. Langley. Smithsonian Contributions to 

 Knowledge, 1891. 



