SECT, xiii.] ACTION OF SUN AND MOON. 109 



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 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 Gibraltar 

 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 is inconsiderable, so that in all probability 

 there is no tide at the poles, or only a small annual and 

 monthly tide. The ebb and flow of the sea are perceptible 

 in rivers to a very great distance from their estuaries. In 

 the Straits 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 magnitude and 

 time. It requires a very wide expanse of water to accumu- 

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

 influence sensible ; on that account the tides in the Medi- 

 terranean and Black Sea are scarcely perceptible. 



These perpetual commotions in the waters are occasioned 

 by forces that bear a very small proportion to terrestrial gra- 

 vitation : the sun's action in raising the ocean is only the 

 35Tigoi3(5 f gravitation 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 ^ 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 moon would 



