Mr. w.) DISSERTATION SECOND. 113 



gravity at the earth's surface, and, therefore, the force 

 with which the sun disturbs the moon's motion is t4t x tst>s 

 of gravity at the earth's surface. This is the solar disturbing 

 force on the moon when distant sixty semidiameters from the 

 earth's centre, but on a body only one semidiameter distant 

 from that centre, that is, on the water of the ocean, the 

 disturbing force would be sixty times less, and thus is 

 found to be no more than yg ^V ootr of gravity at the earth's 

 surface. 



Now, this being the mean force of the sun, is that by which 

 he acts on the waters, 90 degrees distant from the point 

 to which he is vertical, where it is added to the force of gra- 

 vity, and tends to increase the weight and lower the level 

 of the waters. At the point where the sun is vertical, the 

 force to raise the water is about double of this, and, there- 

 fore, the whole force tending to raise the level of the high, 

 above that of the low water, is three times the preceding, 

 or about the T 28 T Vvff<j' °f gravity. Small as this force is, 

 when it is applied to every particle of the ocean, it is capa- 

 ble of producing a sensible effect. The manner in which 

 Newton estimates this effect can only be considered as af- 

 fording an approximation to the truth. In treating of the 

 figure of the earth, he had shown that the centrifugal force, 

 amounting to -^ of gravity, was able to raise the level of 

 the ocean more than seventeen miles, or, more exactly, 

 85,472 French feet. Hence, making the effect proportional 

 to the forces, the elevation of the waters produced by the 

 solar force will come out 1.92 feet. 



But, from the comparison of the neap and spring tides, 

 that is, of the difference and the sum of the lunar and solar 

 forces, it appears, that the force of the moon is to that of the 

 sun as 4.48 to 1. As the solar force raises the tide 1.92 {eet, 

 the lunar will raise it 8.63 feet, so that the two together will 



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