WHAT IS THE TIDE OF THE OPEN ATLANTIC? 475 



the continental shelf attains anything like that depth, but the 

 descent from the shelf on east and west falls rapidly to that fig- 

 ure. The ocean basin is thus slightly larger than the parts left 

 white on this sketch. 



The Atlantic basin is seen to approach much nearer the Span- 

 ish and African coasts than the American or the English and 

 Scandinavian. Sable island, east of Nova Scotia, lies close to 

 the margin of the continental shelf and has its high water 6h. 

 28m. after high water on the west coast of Spain and about two 

 hours before the actual American coast farther west, just as the 

 Spanish coast has its tides earlier than the British isles and 

 northern Europe generally, where a true progressive wave exists 

 and travels across the shallow waters. This oceanic basin is so 

 shaped and proportioned as to possess an oscillation period of 

 half a lunar day, and twice a day the moon's attraction inclines 

 its surface now east, now west. The figures for Sable island and 

 Spain show that low water on the east coincides with high water 

 on the west. As the ocean basin is not bounded by straight lines, 

 every tongue of deep water that advances among shallows toward 

 the land transmits the tidal impulse synchronously with the 

 swaying of the Atlantic. In the shallows progressive waves 

 cany the impulse further. Whole bays respond to the oceanic 

 movement, and only in exceptional areas can cotidals be truly 

 drawn. The Irish channel in Whewells second chart and the 

 Gulf of St Lawrence well illustrate the limitations of the cotidal. 

 The great coastwise ebb and flow of the Atlantic currents govern 

 the long lines of bars and sand islands of the eastern United 

 States. 



It is noteworthy that the. so-called Atlantic ridge, really a 

 broad, gentle swell, must occupy about the same position as the 

 node of the ocean oscillation. One is tempted to speculation on 

 possible accumulations of finest ocean silts in this stiller axis of 

 the swaying mass through the long ages of geologic time. One 

 may wonder again if the moon's periodic impulse does not for- 

 bid a departure of the ocean basin from the form demanded for 

 an oscillation in harmony with lunar time — in other words, 

 whether the moon may not have contributed to the permanence 

 of oceanic basins in governing oceanic tides. The tide must re- 

 sist any attempt to change its period. 



