170 ON AERIAL NAVIGATION. 



created a slight vacuity immediately behind tfce point of «e- 

 paration. The fluid accumulated thus within the cavity 

 has to make its escape at the posterior edge of the surface, 

 •*vhere it is directed considerably downward ; and therefore 

 has to overcome and displace a portion of the direct cur- 

 rent passing with its full velocity immediately below it; 

 hence whatever elasticity this effort requires operates upon 

 the whole concavity of the surface, excepting a small por- 

 tion of the anterior edge. This may or may not be the true 

 theory, but it appears to me to be the most probable ac* 

 count of a phenomenon, which the flight of birds proves to 

 exist. 

 Experiments Six degrees was the most acute angle, the resistance of 

 «?f the French ^j^ was determined by the valuable experiments of the 

 French Academy; and it gave T \ of the resistance, which 

 the same surface would have received from the same cur- 

 ■*• rent when perpendicular to itself. Hence then a superficial 

 foot, forming an angle of six degrees with the horizon, 

 would, if carried forward horizontally (as a bird in the act 

 of skimming) with a velocity of 23*6 feet per second, re- 

 ceive a pressure of T * of a pound perpendicular to itself. 

 And, if we allow the resistance to increase as the square of 

 the velocity, at 27*3 feet per second it would receive a 

 Flight of the pressure of one pound. I have weighed and measured the 

 w^. surface of a great many birds, but at present shall select 



the common rook (corvus frugilegus) because its surface 

 and weight are as nearly as possible in the ratio of a super- 

 ficial foot to a pound. The flight of this bird, during any 

 part of which they can skim at pleasure, is (from an ave- 

 rage of many observations) about 34'5 feet per second. 

 The concavity of the wing may account for the greater re- 

 sistance here received, than the experiments upon plain 

 surfaces would indicate. I am convinced, that the angle 

 made use of in the crow's wing is much more acute than 

 six degrees: but in the observations, that will be grounded 

 ifpon these data, I may safely state, that every foot of such 

 carved surface, as will be used in aerial navigation, will re- 

 ceive a resistance of one pound, perpendicular to itself, 

 tltttW carried through the air in an angle of six degrees with 



the 



