THE VICTORIAN NATURALIST. 51 



and with compression begins to flow out round the edges of the 

 sheet, which then begins to descend with a gradual increasing 

 velocity. If, however, the sheet be in motion in the direction of 

 its plane it may have moved, during that instant of time, from 

 the column of air it was beginning to compress, on to an adjoin- 

 ing column and off this again before it has had time to actually 

 compress it, and thus sail along hardly losing anything in vertical 

 distance. Similarly a bird that, when hanging motionless in the 

 air, would have to extend its feathers to their greatest expanse, so 

 much so that it looks as if there were a strain on its muscles to keep 

 them out when moving, can glide easily through the air, resting 

 as it were on a cushion, without using any expansive strain, and 

 may even lessen its horizontal area — which it does by turning 

 partly on one side — without compressing the underlying air, and 

 so subsiding, at all. 



Now with regard to the bird's means of rising through the air, 

 what can it do? Our thin sheet of metal, or bird, after a little 

 time of suspension over a given point tends to fall. Thus in 

 quite calm air the bird, even to sustain itself, and much more if it 

 wishes to rise, must strike the air with its wings as a fish's tail 

 or a steamer's screw blades strike the water — in order to press 

 them along. Not so, however, if there be a current of air 

 pressing against the bird or sheet of metal, for if either of 

 them be held at an angle to the current of air its weight 

 begins to act as the string of a kite and prevents its being 

 carried backward with the wind ; and the current presses 

 against them because of their inertia, resolving itself into two 

 forces — one pushes against the bird or plate and the other along 

 its plane at right angles to the former, thrusting it upwards or 

 downwards as the slope is towards or away from the direction of 

 the current ; but the bird or thin sheet moves most readily in the 

 direction of the least resistance and it rises more than it goes 

 backward if it hangs at a smaller angle to the vertical than to the 

 horizontal. Of course, if there be an upward current from the 

 surface of the sea the bird will rise in the air while in a horizontal 

 position, and has no need to place its body on an inclined plane. 

 It has also been shown mathematically that when successive 

 strata of air have each successively greater velocities, as happens 

 when the lower currents are stopped by friction of the earth's 

 surface, the bird can acquire a momentum which helps it to rise, 

 but it is difficult to believe that the bird is dependent on these 

 irregular aids for what it has to do every few minutes every day 

 of its life. 



The resultant pressure of the air on the bird's wings and body 

 acts as if the whole were concentrated at a point called the centre 

 of pressure, which, as the bird is symmetrical on either side, 

 lies in a section through the middle of its head, back, and tail, 



