PHYSICAL AND MATHEMATICAL SCIENCES. 327

densities, or specific gravities, of the matter of whichthe several planets are composed. Yet the principle onwhich they are found, is no other than that by whichthe intensity of the earth’s gravitation on the moon isascertained. Gravity is nothing else than weight, pro-portional to the quantity of matter; and density ismeasured by the quantity of matter and the magni-tude.

But this same principle of the mutual attraction be-tween all particles of matter throughout the universe,and the law of gravitation, inversely as the square ofthe distance, and directly proportional to the mass orquantity of matter, was yet found to he applicable to adifferent class of phenomena, and susceptible of prooffrom other facts in the system of the world, of wdiichthey afforded an explanation. Such were the tides ;the general facts of which were investigated by Newton,to a sufficient extent to evince, at least, the general ap-plicability of his principles. He showed that they weredue to the joint action of the sun and moon, and evenestimated the amount to which the tide ought to risefrom theory, supposing its course uninterrupted bycontinents, and the depth uniform. No theory, it isevident, can accurately agree with facts modified bycircumstances so widely different as the actual cases arefrom these suppositions ; but still there is a very nearaccordance between Newton’s result and the averageheight of the tide in the main ocean.

The waters of the sea having a free motion, which thesolid parts of the globe have not, are able to obey theimpulse of attraction to a certain extent, limited bytheir terrestrial gravitation. According to the relativepositions of the two luminaries, the resulting effect oftheir two attractions, conspiring or opposing, produces agreater or less rise in the waters, as each portion comessuccessively under its influence ; and a correspondingrise at the opposite point, since the water on that side,being farther from the sun or moon, is less attractedthan the earth, which is thus, as it were, drawn awayy 4