Scr. XIII. HEIGHT OF THE TIDES. 91 



the Pacific Ocean must be one of the principal sources 

 of our tides ; but, in consequence of the rotation 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 transmitted to 

 the Atlantic, from which sea it moves in a northerly 

 direction along the coasts of Africa and Europe, arriving 

 later and later at each place. This great wave, how- 

 ever, is modified by the tide raised in the Atlantic, 

 which sometimes combines with that from the Pacific 

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

 so that the tides only rise in proportion to their differ- 

 ence. 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 t&ies 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 deter- 

 mined by the mean of a very great number of observa- 

 tions, 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 center of the Pacific they do not exceed one 

 or two feet. The winds have great influence on the 

 height of the tides, according as they conspire with or 

 oppose them ; but the actual effect of the wind in ex- 

 citing the waves of the ocean extends very little below 

 the surface. Even in the most violent storms, the water 

 is probably calm at the depth of ninety or a hundred 

 feet. The tidal wave of the ocean does not reach the 

 Mediterranean nor the Baltic, partly from their position 

 and partly from the narrowness of the Straits of Gib- 

 raltar and of the Categat, but it is very perceptible in 

 the Red Sea and in Hudson's Bay. In high latitudes, 

 where the ocean is less directly under the influence of 

 the luminaries, the rise and fall of the sea w inconsider- 



