VAEIATIO:?^ IN THE TIME OF HIGH WATER 103 



trance to the Soutli Atlantic. It is perhaps better to regard the 

 oscillations which occur at the same time in both hemispheres, as 

 coincident but independent phenomena. 



Nevertheless, in each isolated basin the movements of the sea are 

 much as Whewell has described them. On the coasts of France 

 and the British Isles the tide certainly comes from the open sea, and 

 in its progress along the shores, the original motion which the attrac- 

 tion of the sun and moon produced in the middle of the open sea con- 

 tinually decreases. On penetrating into the shallower seas which 

 surround Ireland and Great Britain, the tidal wave gradually 

 slackens. After having struck Cape Clear and the promontory 

 of Land's End, it is propagated with such slowness around the two 

 islands, that 19 hours elapse before it arrives at the Straits of 

 Dover, where it meets with another wave newer by 12 hours, 

 which has come by the shorter route of the Channel. Whence 

 comes this slackening of the wave ? The researches of astronomers 

 and natural philosophers inform us, that the speed of the tidal wave 

 is proportioned to the depths of the ocean ; driven by an equal 

 force, the circumference of a wheel turns the faster the greater its 

 diameter ; in the same way the tide hastens or slackens its move- 

 ment, according to the depth of the watery mass which it traverses. 

 In those latitudes where the bed of the ocean is 5000 fathoms 

 from the surface, the speed of the wave is about 528 miles an hour ; 

 where the depth is only about 50 fathoms, the tide is not propagated 

 more than about 60 miles in the same space of time ; finally, when 

 the bottom is at about 5 fathoms below the marine surface, the move- 

 ment of the waters is greatly retarded, and does not exceed 15 miles 

 per hour, that is to say, 440 yards per minute. 



In consequence of the delay which the tidal wave experiences, the 

 " establishment," that is to say, the time which elapses between the 

 passage of the moon over the meridian and the moment of full 

 tide, varies singularly in different ports situated near each other. 

 Thus while at Gibraltar there is usually a coincidence between the 

 astronomical and marine phenomena, and the establishment is re- 

 duced in consequence to zero ; this interval is about an hour and 

 15 minutes in the port of Cadiz, and 4 hours at Lisbon. At 

 Bayonne, as at Lorient, it is 3 hours 30 minutes, at the mouth 

 of the Gironde and at Cherbourg it is 7 hours 40 minutes, at 

 Havre 9 hours 15 minutes, at Dieppe 10 hours 40 minutes, at 

 Dunkirk 11 hours 45 minutes. The establishment varies on every 



