52 THE VICTORIAN NATURALIST. 



and as the wings are placed well forward the point on which the 

 pressure acts is well forward in this section. Similarly, its weight 

 acts as if it were all collected at the centre of gravity, situated in 

 the same plane but somewhere about the middle and behind the 

 centre of pressure. Thus the pressure pushes at a point between 

 the head and the centre of gravity. This is what ensures the 

 bird's going head foremost, instead of tail foremost. 



If a bird wishes to rise in the air head uppermost it has to 

 turn its face to the wind, but if with the wind behind it, it must 

 turn head downwards and tail uppermost, which it can and does 

 do perfectly well, though it must involve a more difficult feat of 

 balancing. With a wind behind and its head up, the bird would 

 be driven downwards, and therefore has to assume a nearly 

 horizontal position, or one slightly bending downwards. As the 

 bird has to look for its food on the surface of the water, this is 

 the position it actually does retain on the average. 



A sea bird, then, without flapping its wings has the power of 

 suspending itself in the air, and it has also the power of rising 

 vertically, or vertically with a slight backward movement, if there 

 be a wind ; but how does it do if it wants to move forward 

 against the wind, still without flapping ? 



I must show you, firstly premising that few birds fly much, if 

 ever for more than a short distance, dead against the wind, but 

 almost invariably cross and recross its direction at suitable 

 angles, seldom even forty-five degrees, and more generally not 

 nearer than at about sixty degrees, to its direction, that when 

 it wishes to go against the wind, if without way on, or 

 near the surface of the water, the bird first simply rises 

 vertically by one of the methods above described, or if 

 there be no wind, makes a few strokes with its wings. It 

 then, turning head downwards, slides down the cushion of 

 air at any desired angle, the momentum of its weight 

 carrying it forward and overcoming the pressure of the wind. 

 As it nears the surface of the water, it again turns upward the 

 plane of its wings, as a diver turns his hands up from his out- 

 stretched arms when he wishes to rise to the surface, and is 

 carried on by the same momentum upwards to a level not quite 

 that from which it started, having lost a small vertical space 

 varying with the pressure of the wind against it. It then turns 

 downward, and repeats the process, thus crossing and recrossing 

 the direction of the wind, and either at an interval of several 

 crossings, or at the end of each crossing, recovers the lost 

 vertical distance by one of the methods previously described. 

 This method is the same as that of tobogganing cars, which slide 

 down an incline, and with their momentum go over a smaller 

 hillock, and so on again ; but the friction of the air against the 

 slide, as well as the pressure of the air, stops them. A bird has 



