INTRODUCTION. 15 



ence which such objects cause in the resistance of the air, and 

 that enables him to avoid touching them. 



The horse feels his way by means of the hair ; and birds 

 must in like manner often feel their way by their feathers. 

 Such must be habitually the case with owls and other noctur- 

 nal birds, which can fly darkling through thick woods and 

 other intricate places ; and though the owls have the eyes 

 directed forwards, and not laterally, as many other birds have, 

 they are by that means less capacitated for avoiding by sight, 

 even admitting that they can see with the smallest possible 

 portion of light, those obstacles which it would be the most 

 awkward to encounter those of course which would injure, 

 entangle, or impede their wings. If one wing were to come 

 in contact with a tree, or even with a leaf, the bird would be 

 upset, as certainly as a man is when, in walking heedlessly, 

 he places one foot over a pit or ditch, while the other is on 

 the ground. 



The necessity of feeling with the feathers is not confined 

 to nocturnal birds, but is essential to the safety of all the 

 winged tribes ; the feathers must therefore always be in a 

 state of great perfection. Now, though the shafts of many 

 feathers, and the larger ribs of the webs or vanes of not a few 

 are of considerable substance and strength, all feathers are 

 subdivided till the ultimate ramifications are exceedingly 

 minute. Consequently they produce very large surfaces to 

 the air in proportion to the quantities of matter they contain. 



Feathers are thus very much exposed to atmospheric action, 

 which dries them and renders them unfit for the functions 

 that are required of them. They are also apt to be broken 

 or torn, in the flights, the wars, and the labours of their 

 owners. They are therefore periodically shed and repro- 

 duced; and the reproduction usually takes place in such a 

 way, as that the bird shall be in best feather at the very time 

 when it has the greatest labour to perform. 



