440 Prof. S. P. Langley on th 



le 



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 sup- 

 posed to meet this inclined plane, 

 before the effect of its inertia is ex- 

 hausted, and consequently before it 

 ceases to rise, we have only to suppose 

 the plane to be rotated through 180° 

 about a vertica] 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 supposed to happen with a 

 model inclined plane freely suspended Mg.i. 



in the air, and endowed with the power 



of rotating about a vertical axis so as to change the aspect 

 of its constant inclination, 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 alternate 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 pul- 

 sations 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 

 are in opposite directions, and will produce on a plane, whose 

 inertia enables it to sustain a sensibly uniform motion with 

 the mean velocity of this variable wind, the same lifting 

 effect as if these same alternating winds were in absolutely 

 opposed directions, provided that the constant inclination of 



