ORNITHOLOGIST'S TEXT-BOOK- 71 



use of the word ' infinitely,' unless infinitely small 

 be meant. An Owl to an Owl's bulk of air, is a 

 stone to a pound, as compared with the Dipper's 

 bulk of water to the Dipper ; and if birds rise and 

 descend in the air at pleasure, by the motions of 

 their wings, it is only reversing those motions to 

 enable them to descend or keep themselves down 

 in water. The compression of three feet of water 

 is nearly two pounds and a half on the inch of the 

 feathers, and that, when they are unruffled, as they 

 are in the Dipper, will bring them to very nearly 

 the specific gravity of water. The difference of 

 specific gravity between the bird and the water, is 

 indeed so trifling, that very little effort suffices to 

 move it in any direction, upwards, downwards, or 

 laterally. Birds do not fly upon the principle of 

 specific levity, as with equal wiugs the heavy bird 

 flies best ; they fly because they strike the air more 

 forcibly in the opposite direction to that in which 

 they wish to go, and, under water, the Dipper does 

 just the same : if it wishes to go down, it strikes 

 upward the wings and tail; if to come up, it does 

 just the reverse. The only difference is, that the 

 wings are held ' recovered,' as running birds use 

 them, and that gravitation has even less to do in 

 the matter than in flying. Any one who has ever 

 seen a Dipper under water, or has the slightest 

 knowledge of the mere elements of mechanics, 

 can understand the whole matter in an instant. 

 The Dipper is, indeed, often adduced as an instance 

 of the beautiful simplicity of animal mechanics. 

 The curious habits of the bird will perhaps be an 

 excuse for this digression, which, after all, is more 

 apparent than real. 



" From its not being solely dependent upon at- 

 mospherical temperature for its food, the Dipper is, 

 considering the upland character of its haunts, 

 really an early bird. It sings in January, and its 



