J. Mynde Sculp.
Plate IX.
J. Ferguson delin.
J. Mynde Sculp.
CHAP. XVII.
Of the ebbing and flowing of the Sea.
The cause of the Tides discovered by Kepler.
[PLATE IX].
Their Theory improved by Sir Isaac Newton.
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.
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.