CONTRIVANCE A NECESSITY. 147 



birds whose structure enables them to be adepts in 

 their glorious art. 



It cannot be too often repeated — because mis- 

 conception on this point has been the cardinal 

 error in human attempts to navigate the air — that 

 in all the beautiful evolutions of birds upon the 

 wing, it is weight, and not buoyancy, which makes 

 those evolutions possible. It supplies them, so to 

 speak, with a store of Force which is constant, in- 

 exhaustible, inherent in the very substance of them- 

 selves, and entirely independent of any muscular 

 exertion. All they have to do is to give direction to 

 that internal Force, by acting on the external Force 

 of aerial currents, through the contraction and ex- 

 pansion of the implements which have been given 

 them for that purpose. Those who have watched 

 the flight of birds with any care, must have ob- 

 served that when once they have attained a certain 

 initial velocity and a certain elevation, by rapid 

 and repeated strokes upon the air, they are then 

 able to fly with comparatively little exertion, and 

 very often to pursue their course for long dis- 

 tances without any flapping whatever of the wings. 



