S. P. Langley — Internal Work of the Wind. 55 



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 direction of the wind, but nearly vertically. 

 Presently however, as we recognize, this inertia must be over- 

 come, 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, we have only to 

 suppose the plane to be rotated through 

 180° a.bout 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 fact that its inertia 

 now reappears as an active factor. The 

 annexed sketch (fig. 1) shows a typical 

 representation of what might be sup- — _ 

 posed to happen with a model inclined 

 plane freely suspended in the air, and Fig.i. 



endowed with the power of rotating about 



a vertical axis so as to change the aspect of its constant inclina- 

 tion, which need involve no (theoretical) expenditure of energy, 

 even although the plane possess inertia. We see that this 

 plate would rise indefinitely by the action of the wind in alter- 

 nate directions. 



The disposition of the wind which is here supposed to cause 

 the plane to rise, appears at first sight an impossible one, but 

 we shall next make the important observation that it becomes 

 virtually possible by a method which we shall now point out, 

 and which leads to a practicable one which we may actually 

 employ. 



Figure 2 shows the wind blowing in one constant direction, 

 but alternately at two widely varying velocities or rather (in 

 the extreme case supposed in illustration), where one of the 

 velocities is negligibly small, and where successive pulsations 

 in the same direction are separated by intervals of calm. 



A frequent alternation of velocities, united with constancy 

 of absolute direction, has previously been shown here to be 

 the ordinary condition of the wind's motion ; but attention is 

 now particularly called to the fact that while these unequal 

 velocities may be in the same direction as regards the surface 

 of the earth yet as regards the mean motion of the wind, they 



