WHAT IS THE TIDE OF THE OPEN ATLANTIC? 467 



vertical motion. It is a stationary wave with a central node. 

 As with a pendulum, successive oscillations are in the same 

 period, but the period may be changed by changing the depth 

 of water. If the nodal axis lies north and south, as when the 

 east end of the vessel has been lifted, the motion of the water 

 particles is simultaneously to the west, then simultaneously to 

 the east. A fall on the east corresponds to a rise on the west, 

 the amount of rise and fall depending on distance from the node 

 and (much more) on local configuration. Stationary waves may 

 be studied in a tumbler of water, and the experiment should be 

 tried. 



THE EARLIER VIEW 



It is usual in tidal discussions to assume a general case of con- 

 venient conditions and come later to the real problem — the tides 

 in the case of nature. The general case supposed was a sphere 

 uniformly covered with water. The moon was considered to 

 have the power of heaping up the waters at the points of the 

 earth nearest to itself and farthest away. The deepening of the 

 waters at these two points would be accompanied by a shallow- 

 ing around a circle equatorial to these points as poles. Thus 

 the ocean would assume the shape of a prolate spheroid with 

 longer axis always pointed at the moon. The earth would always 

 have its two high waters at its opposite points, with low waters 

 between. In the mean 6h. 13m. — a half lunar day — would in- 

 tervene between high and low and between low and high. This 

 spheroidal shell would seem to revolve about the earth with the 

 moon, alternately elevating and depressing the water surface of 

 an} r place. The first assumption to reject for the actual world 

 is the earth's uniform envelope of ocean. The Atlantic is barred 

 east and west by continents. The apices of a tidal spheroid can- 

 not come to this water body in a daily swing about the earth. 

 When the moon is over the eastern border of the ocean it might 

 heap the waters there in a tide that would accompany it in its 

 apparent westward path across the ocean; but at the American 

 continent this action must for the moment cease. Each ocean 

 would see the birth and death of a tidal wave at its eastern and 

 western bounds. 



Below the southern continents, in latitude 60°, is a ring of 

 continuous ocean, with tides probably simultaneous, 180° apart.* 

 This belt alone, then, conforms to ideal conditions. It is hard to 



* South Georgia and Aukland island, near this circle, are distant fill. 15m. of longi- 

 tude ; their tides differ in time 9h. 47m. 



