Birds. 6837 



those perpendicular to this course shortest. The objects, therefore, 

 in front will be distinct; the lateral ones distinguished with difficulty. 

 Without, therefore, attempting too vigorous a definition, we may say 

 that the flight of a swallow is on the plan of a straight line or length- 

 ened curve passing through a certain number of points ; and, to accom- 

 plish this, advantage is taken of two great physical laws, — one of 

 dynamics, that a body moving in one direction will acquire an impetus, 

 — and the other a law of optics, that rays of light require a certain 

 time to make a distinct impression on the retina. Now in this mode of 

 taking living prey in the air, wonderful as is its efficacy, there is one 

 condition absolutely necessary to its success : there must be a certain 

 amount of light. If we reduce the light the range of the swallow's 

 sight will be in proportion curtailed till, though he may take one in- 

 sect, he will not be able, at the same time, to have his eye upon one 

 beyond it, and will therefore feed much slower. If the light be 

 still further reduced, the impetus, so important before, will absolutely 

 be in the bird's way, for the range of sight will be so short that it will 

 be carried past the insect before it has time to direct its motion so as 

 to take it. If, on a calm evening, when the piramidigs (Chordeiles) 

 are busy overhead, we go out and attempt to see an insect, even in 

 the most favourable positions, it will be understood how very small an 

 amount of light is requisite to these birds to catch an immense number 

 of their minute prey. It becomes, therefore, very interesting to attempt 

 to trace, as far as observation enables us, the mode in which this new 

 condition of the bird's existence has been'provided for and met. 



" The increased size of the eye and gape do not need comment. 

 This is accompanied also by a great increase in the size of the feathers, 

 by which the bulk of the plumage, relatively to the body of the bird, 

 is rendered much greater than with the swallow. The tail-feathers are 

 much produced, so that though the total length of Chordeiles is half 

 an inch more than that of Acanlhylis, the real length of the body is 

 half an inch shorter. 



" By this arrangement it is obvious that the bird will suffer a great 

 loss in impetus. It could not move at the rate of a swallow, without 

 vastly greater exertion, but in exact proportion it has gained in buoy- 

 ancy, and can turn, stop, move suddenly, laterally, or up or down, in a 

 manner that would require with the latter the most vigorous muscular 

 exertion, impossible to be long continued. The use to which the 

 piramidig puts his new power appears to be this ; the shortening of 

 his range of sight by the diminished light is compensated by the in- 

 creased number of rays that are useful to him. He is moving very 



