CONTRIVANCE A NECESSITY. 175 



result. It is not surprising, therefore, that the 

 muscular movements which serve to turn the 

 axis of a flying bird from one direction to an- 

 other, are very often so extremely minute as 

 generally altogether to elude the sight. But in 

 general terms, it may be said that a bird turns in 

 flying essentially on the same principle as that on 

 which a man turns in walking. It is done in both 

 cases by change in the direction of muscular pres- 

 sure upon a resisting medium. By an exquisite 

 combination of different laws, and by mechanical 

 contrivance in the adjustment of them, it has been 

 given to a bird to find in the thin and yielding air 

 a medium of resistance against which its own mus- 

 cular force may act, as firm and as effective as 

 that which Man finds in the solid earth. 



The Humming Birds are perhaps the most re- 

 markable examples in the world of the machinery 

 of flight. The power of poising themselves in the 

 air, — remaining absolutely stationary whilst they 

 search the blossoms for insects, — is a power essen- 

 tial to their life. It is a power accordingly which 

 is enjoyed by them in the highest perfection. 

 When they intend progressive flight, it is effected 



