372 NATATORES. 



to move the slower the fresher the breeze. They heave and 

 sink, the times being as the square roots of their lengths, so 

 that, if a wave four feet broad changes from ridge to trough 

 in four seconds (which is not very far from the truth), one of 

 sixteen feet will change in eight seconds. Now, as the appa- 

 rent forward motion is half the width, the four-feet wave will 

 appear to move at the rate of rather less than a mile and a 

 half in the hour, the sixteen-feet one at rather less than three 

 quarters of a mile in the hour, which is a very slow motion ; 

 and if one could see one of the mountain seas of the southern 

 ocean on an end view of the waves, they would appear to 

 have hardly any forward motion at all. 



Thus, in the case of single waves, the middle of the slope is 

 a point of rest, on which the sea bird can sit with little more 

 difficulty than on the calm surface. That will, perhaps, be 

 made plainer by the following diagram of 



3 

 A bird at rest on the wave. 



a. I. is the mean level or calm line of the sea, cutting both 

 the black and the dotted curve on the points o. o. 1 is the 

 ridge, and 3 the hollow, at the one end of the vibration ; 

 and 4 the ridge, and 2 the hollow as shown by the dotted 

 line, at the other. The bird at B on the turning point is 

 not moved either up or down ; and as that point is alter- 

 nately on the windward and leeward of the wave, the wave 

 keeps it from drifting in the first case, and affords it shelter 

 in the second. There are some other optical deceptions with 

 regard to the motion of the bird or of anything else that 

 floats on that part of the wave, but they are not easily 



