SENSATION IN FEATHERS. 39 



regions are somewhat abated by the transition to white, from the 

 well-established fact that a surface purely white reflects heat far 

 more copiously than a dark one ; and consequently it is not diffi- 

 cult for us to infer that, in like manner, it prevents any undue 

 waste of the animal heat by radiation. 



The moulting of birds is very gradual, and few of them are 

 ever so bare of feathers as to prevent them from taking wing, 

 and even flying long distances. 



The time of shedding the feathers varies in the different species 

 and in different climes; some moult late in the summer, some in 

 the early autumn, and some in the early spring. 



The summer or autumnal moult is always the most complete; 

 the perennial is generally only a change of color of some portions 

 of the plumage, and not a thorough shedding of tiic feathers. 



Those birds, as well as water-fowl, which extend their migrations 

 far to the North, for the purposes of procreation, receive their 

 fresh plumage after the period of incubation has entirely passed 

 by, so that they come out fresh and entirely freed from all the 

 filth and vermin which their previous sedentary occupations may 

 have entailed on them. 



