The Flight of Birds and the Mechanics of Flight. 421 



to the waves as possible, often seeming to stroke the hack of the 

 wave with the tip of the wing. I have seen large gulls of the 

 albatross kind advance rapidly in the teeth of the wind, close to the 

 surface of the water, without a single stroke of the wing for a hun- 

 dred yards or more. The bird was, in fact, gliding down an inclined 

 plane. From the momentum thus acquired the ]:)ird can lift itself 

 to a considerable height above the waves, from which it descends by 

 an easy slope close to the surface again. The bird merely steers 

 itself in the breeze, which does all the work. So little effort being 

 required from the bird, we can understand how it can keep on the 

 wing for so long. This also seems to explain why it is that alba- 

 trosses and similar soaring gulls are so seldom seen to alight upon 

 the water to feed. If severe work were done fuel, in the shape of 

 food, would be constantly needed. Consider now tlie flight of the 

 gannet. This bird is not a soaring bird ; it flaps its wings strongly 

 and raises itself to a considerable height above the waves where it 

 can obtain very little support from upward currents, and remains 

 there until it plunges into the sea for its prey. It then, with great 

 effort, raises itself to the necessary height, and generally, as I have 

 noticed, within five minutes makes another plunge. It is, in fact, 

 feeding, for it must require a good deal of food to enable it to make 

 such violent 'exertion. As it cannot rest in the air like the albatross, 

 it is frequently to be seen seated on the water. 



About the flight of flying-flsh there is controversy as to whether 

 the animal beats the air or soars. To ordinary eyesight — at any rate 

 to mine — there appears to be no vibratory motion. Some observers, 

 however, are confident that the wings do vibrate, and argue that 

 failure to observe it is due to the rapidity of the motion. But 

 rapidity of motion results in invisibilit}", as is the case with the wings 

 of insects and Immming birds. It is again urged in favour of the 

 vibratory theory that it is incredible that an animal could jump for 

 one or two hundred yards. This jump, ho.wever, is easily explained 

 by the theory of upward currents. Directly the fisli springs from 

 the water he commences to soar as an albatross does, and is carried 

 forward by the upward currents, of which he obtains full benefit, 

 being close to the surface of the w^ater during most of his flight. If 

 he touch the top of a wave he sculls with his tail and obtains fresli 

 impetus ; and perhaps he might take very much longer flights than 

 he does if he were more independent of salt water. 



Ordinary gulls, which seldom soar, will do so when conditions are 

 favourable, as when following a vessel they happen to find them- 

 selves in the upward current produced by the glancing of the wind 

 from the vessel's side. I have seen gulls resting, without apparent 



