II. THE SWALLOW. 



77 



section of the wing, thick towards the bird's 

 head, and curved 'hke a sickle, so that in 

 striking 'down it catches the air, hke a reaping- 

 hook, and in rising up, it throws off the air 

 Hke a pent-house. 



70. The stroke would therefore be vigorous, 

 and the recovery almost effortless, were even 

 the direction of both actuall}' vertical. But 

 they are vertical only with relation to the 



Fig. s. 



^- 



■^ 



bird's body. In space they follow the forward 

 flight, in a softly curved line ; the downward 

 stroke being as effective as the bird chooses, 

 the recovery scarcely encounters resistance in 

 the softly gliding ascent. Thus, in Fig. 5, 

 (I can only explain this to readers a little 

 versed in the elements of mechanics,) if B is 

 the locus of the centre of gravity of the bird, 

 moving in slow flight in the direction of the 



