WHAT IS THE TIDE OF THE OPEN ATLANTIC? 473 



cotidal line is to be regarded as the crest-line of a great wave, 

 sweeping from shore to shore, as it might be seen by an eye far 

 above the earth. The characteristic feature of such a wave is 

 that every point of the ocean is regarded as first rising, then 

 falling. Such was probably Whewell's conception, and it is wide- 

 spread toda}^ ; yet with the abundant data of today it is not pos- 

 sible to comprehend how a progressing wave should adapt itself 

 so completely to the shores as is found to be the case. An ad- 

 vancing wave would doubtless tend to adjust itself to the shores 

 of an estuary, but the adjustment observed is more than a 

 tendency. 



Opposed to this conception is .that of a stationary wave, con- 

 ceived to have a medial point without vertical motion, called a 

 node. Contemporaneous with a rise of water on one side of this 

 node occurs a fall on the other. For the ocean there is no pro- 

 gression of high water ; the whole water body swashes alternately 

 east and west. For an ocean to oscillate about a node in adjust- 

 ment to the moon's apparent motion is only possible with a 

 given relation between depth and width. By counting the os- 

 cillations in 5 or 10 seconds with various depths of water in a 

 bowl or tumbler, the reader may satisfy himself that for each 

 combination of width and depth there is a constant period of 

 oscillation. If the North Atlantic has such an oscillation in a 

 period of a lunar half day, it must have the width and depth 

 that correspond. 



GROWTH OF THE LATER VIEW 



The first suggestion of such an oscillation was by Young:* 

 " We may therefore consider the Atlantic as a detached sea about 

 3,500 miles long and 3 miles deep." The depth he assumes from 

 theoretical considerations. He considers that the wave from the 

 southern ocean might meet the local oscillation about Gibraltar, 

 when it would doubtless superpose itself upon it. The moon's 

 relation to the motion of the detached ocean is thus suggested 

 by Dr Young : t "The oscillations of the sea, . . . consti- 

 tuting the tides, are subject to laws exactly similar to those of 

 pendulums capable of performing similar vibrations in the same 

 time and suspended from points which are subjected to . . . 

 regular vibrations, of which the . . . periods are completed 



* Natural Philosophy, vol. 1, p. 581. 



t Nicholson's Journal, 1813, August, p. 217. 



