CONTRIVANCE A NECESSITY. 157 



The mechanical laws which are appealed to in 

 all these modifications of structure require adjust- 

 ments of the finest kind, and some of them are 

 so curious and so beautiful that it is well worth 

 following them a little further in detail. 



There are two facts observable in all Birds of 

 great and long-sustained powers of flight : — the 

 first is, that they are always provided with wings 

 which are rather long than broad, sometimes 

 extremely narrow in proportion to their length ; 

 the second is, that the wings are always sharply 

 pointed at the ends. Let us look at the mechani- 

 cal laws which absolutely require this structure for 

 the purpose of powerful flight, and to meet which 

 it has accordingly been devised and provided. 



One law appealed to in making wings rather 

 long than broad is simply the law of leverage. 

 But this law has to be applied under conditions of 

 difficulty and complexity, which are not apparent 

 at first sight. The body to be lifted is the very 

 body that must exert the lifting power. The force 

 of gravity which has to be resisted may be said to 

 be sitting side by side, occupying the same parti- 

 cles of matter, with the vital force which is to give 



