6S40 



Birds. 



is well known, the almost constant habit of the Himndinidse is to make 

 a very rapid wheel, one wing much depressed, so that the whole ex- 

 panse takes an oblique direction. The object of this is sufficiently 



obvious, viz. to reverse the direc- 

 tion of flight with the least pos- 

 sible exertion and the greatest 

 possible economy of impetus. 

 We know that in skating, and 

 many other examples besides 

 from theory, there is none so 

 efficacious as this. 



But the piramidig, as has been 

 \ before shown, has no impetus, 

 ■^ for he has been dashing about in 

 every direction, and if he has 

 had no reason for preserving it, 

 and consequently goes the sim- 

 plest way to work, by a single 

 blow of the wings bolts round at an angle. It is this angular turn 

 that is one of the chief distinctions between the flight of the two 

 birds. Now let us suppose that a piramidig enters the swarm at the 

 point A of the circle T have drawn : he zigzags, tumbles and jerks in 

 a tolerably straight line till he arrives at b. Here he finds himself at 

 the edge of the swarm ; the insects become very scattered, so he bolts 

 round at an angle ; but it is of course of some consequence what sort 

 of an angle this is. Suppose he did not turn enough — made the angle 

 too obtuse — it is obvious that he would go out of the swarm to d ; if the 

 angle were too acute, that, like a swallow, he would return nearly on 

 his former course. But this he does not do, and w^e can easily under- 

 stand that his dashes and flutterings have created far more havoc and 

 dismay than the rapid glide of the other: he therefore takes any other 

 angle between these two. Let us suppose a right angle or something 

 like it. He dashes on in the same way till he comes to the point c. 

 Now it is plain if he made a right angle again it would take him out 

 of the circle ; he therefore makes any angle less than a right angle, and 

 arrives at A again. Of course these angles may be constantly varied, 

 but they will tend to equal two right angles. But, I would remark, 

 this mode of flight is by no means constant, and can be only occa- 

 sionally observed, perhaps partly from the difficulty of keeping in the 

 mind the bird's previous courses ; but it will be seen, that. — provided 

 the requisite data be present, — 



