152 THE REIGN OF LAW. 



And here, indeed, we open up a new branch of 

 the same inquiry, showing, in new aspects, how the 

 universality and unchangeableness of all natural 

 laws are essential to the use of them as the in- 

 struments of Will ; and how by being played off 

 against each other they are made to express every 

 shade of thought, and the nicest change of pur- 

 pose. The movement of all flying animals in the 

 air is governed and determined by Forces of mus- 

 cular power, and of aerial resistance and elas- 

 ticity being brought to bear upon the Force of 

 Gravity, whereby, according to the universal laws 

 of motion, a direction is given to the animal which 

 is the resultant, or compromise between all the 

 Forces so employed. Weight, as we have seen, is 

 one of these Forces — absolutely essential to that 

 result, and no flying animal can ever for a moment 

 of time be buoyant, or lighter than the air in which 

 it is designed to move. But it is obvious that, 

 within certain limits, the proportion in which these 

 different Forces are balanced against each other, 

 admits of immense variety. The limits of variation 

 can easily be specified. Every flying animal must 

 have muscular power great enough to work its 



