295. The cause of the Tides was discovered by KEPLER, who, in his _Introduction to the Physics of the Heavens_, thus explains it: “The Orb of the attracting power, which is in the Moon, is extended as far as the Earth; and draws the waters under the torrid Zone, acting upon places where it is vertical, insensibly on confined seas and bays, but sensibly on the ocean whose beds are large, and the waters have the liberty of reciprocation; that is, of rising and falling.” And in the 70th page of his _Lunar Astronomy_——“But the cause of the Tides of the Sea appears to be the bodies of the Sun and Moon drawing the waters of the Sea.” This hint being given, the immortal Sir ISAAC NEWTON improved it, and wrote so amply on the subject, as to make the Theory of the Tides in a manner quite his own; by discovering the cause of their rising on the side of the Earth opposite to the Moon. For KEPLER believed that the presence of the Moon occasioned an impulse which caused another in her absence. [Sidenote: Explained on the Newtonian principles. Fig. I. Fig. I.] 296. It has been already shewn § 106, that the power of gravity diminishes as the square of the distance increases; and therefore the waters at _Z_ on the side of the Earth _ABCDEFGH_ next the Moon _M_ are more attracted than the central parts of the Earth _O_ by the Moon, and the central parts are more attracted by her than the waters on the opposite side of the Earth at _n_: and therefore the distance between the Earth’s center and the waters on it’s surface under and opposite to the Moon will be increased. For, let there be three bodies at _H_, _O_, and _D_: if they are all equally attracted by the body _M_, they will all move equally fast toward it, their mutual distances from each other continuing the same. If the attraction of _M_ is unequal, then that body which is most strongly attracted will move fastest, and this will increase it’s distance from the other body. Therefore, by the law of gravitation, _M_ will attract _H_ more strongly than it does _O_, by which, the distance between _H_ and _O_ will be increased: and a spectator on _O_ will perceive _H_ rising higher toward _Z_. In like manner, _O_ being more strongly attracted than _D_, it will move farther towards _M_ than _D_ does: consequently, the distance between _O_ and _D_ will be increased; and a spectator on _O_, not perceiving his own motion, will see _D_ receding farther from him towards _n_: all effects and appearances being the same whether _D_ recedes from _O_ or _O_ from _D_. [Sidenote: PLATE IX.]