34 THE BONES. 



part white, but in other respects they differ materially from 

 those of four-footed animals, being composed of a thin, firm, 

 and partly elastic substance, formed in layers apparently 

 fastened together, and almost always hollow ; the cavities 

 never containing marrow, but air, and communicating with the 

 lungs by considerable openings; whereby they are rendered 

 buoyant and light to a much greater degree than is generally 

 supposed. Thus, a portion of the leg of a Goose, about two 

 inches in length, weighed about forty grains, while a piece of 

 the leg of a rabbit (the marrow having been extracted, and 

 both being perfectly dry, and as nearly as possible of the same 

 thickness and length), weighed seventy-five grains, or nearly 

 twice the weight of the similarly-sized bone of the Goose ; 

 and yet so firm and strong was this latter, that although in 

 diameter it was less than one-eighth of an inch, and the solid 

 tubular part not more than one-hundredth part of an inch in 

 thickness, it could not be broken asunder by the hand. 



It is upon this principle mechanics and engineers act in con- 

 structing strong supports, knowing that if any quantity of 

 material is to be fabricated into a rod of a certain length, the 

 rod will be strong in proportion to its thickness ; and that if 

 the figure remains the same, that thickness can only be in- 

 creased by making it hollow. Therefore, hollow rods or tubes 

 of the same length and quantity of matter have more strength 

 than solid ones of less diameter. This is but one out of the 

 hundreds of instances in which the wisdom of man has been 

 perfected by studying the mode by which the great Creator 

 accomplishes His purposes. 



It is evident that a creature thus furnished with bones so 

 much lighter than those of other animals, must have a prodi- 

 gious advantage in raising itself in the air. But besides this 

 superior lightness of the skeleton, these bones, from their 

 hollow structure, act as pipes, supplying air in abundance ; 

 and thus not only rendering the bird still lighter, but enabling 

 it to breathe at heights at which a human being would be 

 gasping for breath. Travellers who have ascended very high 

 mountains find, that when they get near the summits, the air 



