134 - OF THE NATURE OF THE TIDES.

served, that if all the parts of the earth wereequally attracted, they would suffer but littlechange in their mutual situations.

That this doctrine may be still more clearlyunderstood, let it be considered, that thoughthe earth’s diameter bears a considerable pro-portion to the distance of the earth from themoon, yet this diameter is almost nothing whencompared to the distance of the earth from thefun. The difference of the fun’s attraction,therefore, on the sides of the earth under andopposite to him, will be much less than thedifference of the moon’s attraction on the sidesof the earth under and opposite to her: and, forthis reason, the moon must raise the tides muchhigher than they can be raised by the sun.Newton has calculated the effect of the fun’sinfluence, in this cafe, and found that it is aboutthree times less than that of the moon. Theaction of the fun alone would, therefore, besufficient to produce a flux and reflux of thesea; but the elevations and depressions occasionedby this means, would be about three times lessthan those produced by the moon.

The tides, then, are not the sole production ofthe moon, but of the joint forces of the fun andmoon together. Or, properly speaking, thereare two tides, a solar one, and a lunar one;which have a joint or opposite effect, accordingto the situation pf the bodies which produce

them.