126 L. HARGRAVE. 



bers of birds follow the vessel and make wide circuits on either 

 side of the wake ; their interest is centered on garbage, and their 

 efforts are directed to keeping astern, their weight and area are 

 such that they must keep moving through the air at a nearly 

 uniform speed in order that they may be supported ; this velocity 

 I estimate at forty miles per hour. 



If you direct your attention to the position of a bird with 

 regard to the wave surface, it will speedily be noticed to be nearly 

 always on the rising side or face of the wave and moving apparently 

 at right angles to the wave's course, but really diagonal to it. 



The bird is going to leeward as fast as the wave is; and, if that 

 speed is too great for its requirements it turns towards the crest, 

 points one wing to the sky and uses its velocity to shoot upwards 

 high above the back of the wave, and then descends to the trough 

 of the following wave along the face of which it glides : the back 

 of the wave is its peculiar aversion. Now there has been no 

 flapping and the performance takes place with or without wind, 

 all the bird requires is the wave. 



As to the effect of the wave on the air, we will suppose the 

 water to be quite flat and the air motionless, a heavy undulation 

 comes on the scene, it has to pass, so it pushes the air up with its 

 face, letting it fall again as its back glides onwards. The air on 

 the face is slightly compressed, that on the back lowered in pressure, 

 both operations taking power out of the wave and eventually 

 largely contributing to its extinction. 



The closer the bird is to the surface of the water, the firmer 

 and more inelastic is the uplift of the rising air. The bird appears 

 to almost feel the surface with the tip of its weather wing. 



The case I wish you to consider is that of a sea-wave, for 

 example one hundred and eighty feet long and ten feet high, 

 travelling at eighteen knots, or say, thirty feet per second under 

 calm air. This wave will raise all the air as it passes ten feet, at 

 the mean rate of three and one-third feet per second. 



The rate will vary from zero in the trough, attaining its maxi- 

 mum velocity at half the wave height, or where the wave is 



