150 THE REIGX OF LAW. 



from a great height above it. I have seen a 

 Merlin dash down from a great distance with its 

 wings so closed as to seem almost wholly folded. 

 The Gannet in diving for fish does not close its 

 wings at all, but turning them and the whole 

 axis of its body into the perpendicular, and thus 

 allowing its great weight to act without any coun- 

 teraction, dashes itself into the sea with foam. But 

 every variety of forward motion is attained by 

 different degrees of contraction and exposure, 

 according to the strength of the breeze with which 

 the bird has to deal. The limit of its velocity is 

 the limit of its momentum, and the limit of its 

 momentum is the limit of its weight. The lisfht- 

 ness of a bird is therefore a limit to its velocity. 

 The heavier a bird is, the greater is its possible 

 velocity of flight — because the greater is the store 

 of force — or, to use the language of modern 

 physics, the greater is the quantity of " potential 

 energy" — which, with proper implements to act 

 upon aerial resistance, it can always convert into 

 upward, or horizontal, or downward motion, ac- 

 cording to its own management and desires. 



It will be at once seen from this view of the 



