318 INSECTIVOR^:. 



out o' their wits," because there are " impossible cases" in 

 that problem), how the dipper can contrive to keep " beneath 

 a fluid so infinitely more dense than itself." That is a 

 strange 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 wings 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 know- 

 ledge 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 appa- 

 rent than real. 



From its not being solely dependent upon atmospherical 

 temperature for its food, the dipper is, considering the upland 



