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



and intelligence, and that there may thus be a sense in which 

 human flight may be possible, although flight depending 

 wholly upon the action of human muscles be forever impos- 

 sible. 



Let me resume the leading points of the present memoir in 

 the statement that it has been shown : 



(1) That the wind is not even an approximately uniform 

 moving mass of air, but consists of a succession of very brief 

 pulsations of varying amplitude, and that, relatively to the 

 mean movement of the wind, these are of varying direction. 



(2) That it is pointed out that hence there is a potentiality 

 of " internal work " in the wind, and probably of a very great 

 amount. 



(3) That it involves no contradiction of known principles to 

 declare that an inclined plane, or suitably curved surface, 

 heavier than the air, freely immersed in, and moving with the 

 velocity of the mean wind, can, if the wind pulsations here 

 described are of sufficient amplitude, and frequency, be sus- 

 tained or even raised indefinitely without expenditure of 

 internal energy, other than that which is involved in changing 

 the aspect of its inclination at each pulsation. 



(4) That since (A) such a surface, having also power to 

 change its inclination, must gain energy through falling during 

 the slower, and expend energy by rising during the higher, 

 velocities ; and that (B) since it has been shown that there is 

 no contradiction of known mechanical laws in assuming that 

 the surface may be sustained or may continue to rise indefi- 

 nitely, the mechanical possibility of some advance against the 

 direction of the wind, follows immediately from this capacity 

 of rising. It is further seen that it is at least possible that 

 this advance against the wind may not only be attained rela- 

 tively to the position of a body moving with the speed of the 

 mean wind, but absolutely, and with reference to a fixed point 

 in space. 



(5) The statement is made that this is not only mechanically 

 possible, but that in the writer's opinion, it is realizable in 

 practice. 



Finally, these observations and deductions have, it seems to 

 me an important practical application not only as regards a liv- 

 ing creature like the soaring bird but still more as regards a 

 mechanically constructed body, whose specific gravity may 

 probably be many hundred or even many thousand times that 

 of the atmosphere. We may suppose such a body to be sup- 

 plied with fuel and engines, which would be indispensable to 

 sustain it in a calm, and yet which we now see might be ordi- 

 narily left entirely inactive, so that the body could supposably 

 remain in the air and even maintain its motion in anv direc- 



