OP TIDES. 333 



If the earth were covered all over with the sea to a great depth, 

 the tides would be regularly subservient to these laws ; but various 

 causes combine to produce a great diversity of effect, according to 

 the peculiar situation and circumstances of places, shoals, fords, 

 and straits : thus, a slow and imperceptible motion of a large body 

 of water, suppose two miles deep, will be sufficient to elevate its 

 surface ten or twelve feet in a tide's time ; whereas, if the same 

 quantity of water is forced through a narrow channel forty or fifty 

 fathoms deep, it produces a very rapid stream, and of course the 

 tide is found to set strongest in those places where the sea grows 

 .narrowest, the same quantity of water being constrained to pass 

 through a smaller passage, as in the straits between Portland and 

 Cape la Hogue in Normandy ; and it would be still more so between 

 Dover and Calais, if the tide coming round the island did not 

 check it. 



The shoalness of the sea and the intercurrent continents are the 

 reasons why the tides in the open ocean rise but to very inconsider. 

 able heights, when compared to what they do in wide-mouthed 

 rivers opening in the direction of the stream of the tide; and that 

 high water is some hours after the moon's appulse to the meridian,, 

 as it is observed upon all the western coast of Europe and Africa 

 from Ireland to the Cape of Good Hope ; in all which a south- 

 west moon makes high water ; and the same is said to be the case 

 on the western coast of America : so that tides happen to different 

 places at all distances of the moon from the meridian, and conse- 

 quently at all hours of the day. 



To allow the tides their full motion, the space in which they ar 

 produced ought to extend from east to west 9^ at least ; such be- 

 ing the distance between the places most raised and depressed by 

 the moon's influence. Hence it appears that such tides can only 

 be produced in large oceans, and why those of the Pacific exceed 

 those of the Atlantic ocean : hence also it is obvious why the tides 

 in the torrid zone between Africa and America, where the ocean is 

 narrower, are exceeded by those of the temperate zones on either, 

 side : and hence we may comprehend why the tides are so small in 

 islands at a great distance from the shores, since the water cannot 

 rise on one shore without descending on the other ; so that at the 

 intermediate islands it must continue at a mean height between its 

 elevations on those shores 



