98 HEIGHT OF THE TIDES. SECT. XIII. 



do for many ages. Thus the mean distance of the moon and the 

 consequent minute increase in the height of the tides will oscil- 

 late between fixed limits for ever. 



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 they conspire with or oppose them. But the 

 actual effect of the wind in exciting 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 fathoms. 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 Gibraltar and of the 

 Categat, but it is very perceptible in the Eed Sea and in Hudson's 

 Bay. The ebb and flow of the sea are perceptible in rivers to a 

 very great distance from their estuaries. In the Narrows of Pauxis, 

 in the river of the Amazons, more than five hundred miles from 

 the sea, the tides are evident. It requires so many days for the 

 tide to ascend this mighty stream, that the returning tides meet a 

 succession of those which are coming up ; so that every possible 

 variety occurs at some part or other of its shores, both as to mag- 

 nitude and time. It requires a very wide expanse of water to 

 accumulate the impulse of the sun and moon, so as to render their 

 influence sensible ; on that account the tides in the Mediterranean 

 and Black Sea are scarcely perceptible. 



These perpetual commotions in the waters are occasioned by 

 forces that bear a very small proportion to terrestrial gravitation : 

 the sun's action in raising the ocean is only the 3 g44g 66 f gravita- 

 tion at the earth's surface, and the action of the moon is little 

 more than twice as much ; these forces being in the ratio of 

 1 to 2'35333, when the sun and moon are at their mean distances 

 from the earth. From this ratio the mass of the moon is found 

 to be only the 7 3 part of that of the earth. Had the action of the 

 sun on the ocean been exactly equal to that of the moon, there 

 would have been no neap tides, and the spring tides would have 

 been of twice the height which the action of either the sun or 



