140 THE REIGN OF LAW. 



equal force, it must produce equal resistance, — an 

 equal rebound from the elasticity of the air. If 

 this difficulty were not evaded somehow, flight 

 would be impossible. But it is evaded by two 

 mechanical contrivances, which, as it were, triumph 

 over the laws of aerial resistance by conforming to 

 them. One of these contrivances is that the upper 

 surface of the wing is made convex, whilst the 

 under surface is concave. The enormous differ- 

 ence which this makes in atmospheric resistance is 

 familiarly known to us by the difference between 

 the effect of the wind on an umbrella which is 

 exposed to it on the under or the upper side. The 

 air which is struck by a concave or hollow surface, 

 is gathered up, and prevented from escaping, 

 whereas the air struck by a convex or bulging 

 surface escapes readily on all sides, and compara- 

 tively little pressure or resistance is produced. 

 And so, from the convexity of the upper surface 

 of a bird's wing, the upward stroke may be made 

 with comparatively trifling injury to the force 

 gained in the downward blow. 



But this is only half of the provision made 

 against a consequence which would be so fatal to 



