232 WITH THE U. S. NATURALISTS 



''Not especially, sir," answered the boy. 



''Well, you should. If you look at the skeleton 

 of a bird, you'll see that it is built like a boat's 

 hull, but with a very deep keel, like the keel of a 

 yacht, the curves tapering forward to the neck 

 and back to the tail. And, just as a yacht has 

 smooth sides and is painted so that there is not a 

 single projection to catch the water, so a bird's 

 body is fitted with close-fitting feathers all point- 

 ing backwards towards the tail. The muscles 

 which work the wings lie on either side of this 

 great keel or breastbone. 



"Each wing is made of three parts, arranged 

 like the letter 'Z,' the top of which corresponds to 

 the upper arm, the downward stroke to the fore- 

 arm and the lower horizontal stroke to the hand. 

 Birds have only three fingers, and of these the 

 thumb is very small and the other two fingers are 

 bound together to form a rigid rod. So much for 

 the machinery of the wing, Shan. 



"On each of these three parts of the 'Z' there 

 are feathers. Those on the hand we call 'pri- 

 maries,' and those on the fore-arm we call 'sec- 

 ondaries.* These do most of the work. The 

 feathers on the upper arm are called 'tertiaries,' 

 and though they are not of much value in flight for 



