108 HEIGHT OF THE TIDES. [SECT. xirr. 



sion of them follow one another in a north-westerly direc- 

 tion down the Pacific and Atlantic Oceans, modified, as 

 they proceed, by the depth of the water and form of the 

 coasts. When a tidal wave leaves the Antarctic Ocean and 

 enters the Pacific, it moves along the western coast of 

 America to its farthest end, whilst it is so much obstructed 

 by the number of islands in the middle of that ocean that 

 it is hardly perceptible. But, in consequence of the rota- 

 tion of the earth and the inertia of the ocean, high water 

 does not happen till some time after the moon's southing 

 (N. 155). The tide raised in that world of waters is trans- 

 mitted to the Atlantic, from which sea it moves in a north- 

 erly direction along the coasts of Africa and Europe, arriv- 

 ing later and later at each place. This great wave however 

 is modified by the tide raised in the Atlantic, which some- 

 times combines with that from the Antarctic Ocean in 

 raising the sea, and sometimes is in opposition to it, so that 

 the tides only rise in proportion to their difference. This 

 vast combined wave, reflected by the shores of the Atlantic, 

 extending nearly from pole to pole, still coming northward, 

 pours through the Irish and British Channels into the 

 North Sea ; so that the tides in our ports are modified by 

 those of another hemisphere. Thus the theory of the tides 

 in each port, both as to their height and the times at which 

 they take place, is really a matter of experiment, and can 

 only be perfectly determined by the mean of a very great 

 number of observations, including several revolutions of 

 the moon's nodes. 



The height to which the tides rise is much greater in 

 narrow channels than in the open sea, on account of the 

 obstructions they meet with. The sea is so pent up in the 

 British Channel that the tides sometimes rise as much as 

 fifty feet at St. Malo on the coast of France ; whereas on the 

 shores of some of the South Sea islands, near the centre of 

 the Pacific, they do not exceed one or two feet. The winds 

 have great influence on the height of the tides, according as 



