428 THK MECHANICAL FUNCTIONS, 



terrestrial animals. The order Cetacea comprises 

 by far the largest animals which inhabit the globe. 

 Whatever may have been the magnitude of those 

 huge monsters which once moved in the bosom of 

 the primeval ocean, or stalked with gigantic strides 

 across antediluvian plains, and whose scattered 

 remains bear fearful testimony of the convidsions 

 of a former world, certain it is that, at the present 

 day, the Whales of the northern seas are the most 

 colossal of the living animal structures existing on 

 the surface of this planet. 



A cursory survey of the organization of the tribes 

 belonging to this semi-amphibious family, will im- 

 press us with the resemblance they bear to fishes ; 

 for they present the same oval outline of the body ; 

 the same compact form of the trunk, which is 

 united with the head without an intervening neck ; 

 the same fin-like shape of the external instruments 

 of motion; and the same enormous expansion and 

 prolongation of the tail, which is here also, as in 

 fishes, the chief agent in progression. With all 

 this agreement in external characters, their internal 

 economy is conducted on a totally different plan ; 

 for although constantly inhabiting the ocean, their 

 vital or2:ans are so constructed as to admit of their 

 breathing only the air of the atmosphere ; and the 

 consequences which flow from this difference are 

 of great importance. The necessity of aerial respi- 

 ration compels them to rise, at short intervals, to 

 the surface of the water ; and this air, with which 

 they fill their lungs in respiration, gives their 

 bodies the buoyant force which is required to facili- 

 tate their ascent, and supersedes the necessity of 



