92 ACTION OP SUN AND MOON. SECT. XIII. 



able, 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 vari- 

 ety occurs at some part or other of its shores, both as 

 to magnitude 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 occa- 

 sioned by forces that bear a very small proportion to 

 terrestrial gravitation : the sun's action in raising the 

 ocean is only the ^^ r VrroT 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 dis- 

 tances 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 have produced separately ; a phenomenon 

 depending upon the interference of the waves or undu- 

 lations. 



A stone plunged into a pool of still water occasions a 

 series of waves to advance along the surface, though the 

 water itself is not carried forward, but only rises into 

 heights and sinks into hollows, each portion of the sur- 

 face being elevated and depressed in its turn. Another 

 stone of the same size thrown into the water near the 

 first, will occasion a similar set of undulations. Then if 

 an equal and similar wave from each stone arrive at the 

 same spot at the same time, so that the elevation of the 

 one exactly coincides with the elevation of the other, 

 their united effect will produce a wave twice the size of 



