SECTION TENTH.

On the Ebbing and Flowing of the Sea.

THE cause of the tides was first discovered by Kepler, who 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 larger, and the waters have the liberty of reciprocation, that is of rising and falling. And in the 70th page of his lunar Astronomy he says: 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 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.

It has been already mentioned, that the power of gravity diminishes as the square of the distance increases, and therefore the waters on the side of the earth, next the Moon, are more attracted than the central parts of the earth by the Moon, and the central parts are more attracted by her, than the waters on the opposite side of the earth. Therefore the distance between the earth's centre, and the water, or its surface, will be increased. If the attraction be unequal, then that body which is most strongly attracted. will move with greater rapidity, and this will increase its distance from the other body.

As this explanation of the ebbing and flowing of the Sea is deduced from the earths' constantly falling toward the Moon by the power of gravity, some may find a difficulty in conceiving how this is possible when the Moon is full, or in opposition to the Sun; since the earth revolves about the Sun, and must continually fall towards it; and therefore cannot fall contrary ways at the same time: or if the earth is constantly falling towards the Moon, they must come together at last. To remove this difficulty, let it be considered that it is not the centre of the earth that describes the annual orbit round the Sun, but the common

centre of gravity of the earth and Moon together; and that while the earth is moving round the Sun, it also describes a circle around that centre of gravity, going as many times around it in one revolution about the Sun, as there are lunations, or courses of the Moon around the earth; is constantly falling towards the Moon from a tangent to the circle it describes, around the said centre of gravity.

The influence of the Sun in raising the Tides, is but small in comparison of the Moon's though the earth's diameter bears a considerable proportion to its distance from the Moon, it is next to nothing when compared to its distance from the Sun. Therefore, the difference of the Sun's attraction on the sides of the earth, under, and opposite to him, is much less than the difference of the Moon's attraction on the sides of the earth under, and opposite to her: therefore the Moon must raise the Tides much higher than they can be raised by the Sun. On this theory, (so far as it has been explained,) the Tides ought to be the highest directly under, and opposite to the Moon; that is, when the Moon is due north or south. But we find in open Seas, where the water flows freely, the Moon is gen

* This centre is as much nearer the earth's centre, than the Moons, as the earth is heavier, or contains a greater quantity of matter than the Moon which is about 40 times. If both bodies were suspended from it, they would hang in equilibrio. Therefore divide the Moon's distance from the earth's centre, (240,000 miles,) by 40, and the quotient, will be the distance from the centre of the earth to the centre of gravity, which is 6,000 miles, or 2,000 from the earth's surface.

erally past the north and south meridian, when it is high water. The reason is obvious, were the Moon's attraction to cease wholly when she was past the meridian, yet the motion of ascent communicated to the water before that time, would make it continue to rise for some time afterward, much more must it continue to rise, when the attractiou is only diminished. A little impulse given to a moving ball, will cause it to move further than it otherwise would have done. Or, as experience shows that the weather in Summer, is warmer at 2 o'clock in the afternoon, than when the Sun is on the meridian, because of the increase made to the heat already imparted.

The Tides do not always answer to the same distance of the Moon from the meridian at the same places, but are variously affected by the action of the Sun, which brings them on sooner when the Moon is in her first and third quarters, and keeps them back later when she is in her second and fourth; because, in the one case, the Tide raised by the Sun alone, would be earlier than the Tide raised by the Moon, and in the other case later.

The Moon goes round the earth in an elliptical orbit, and therefore in every lunar month she approaches nearer to the earth than her mean distance, and recedes further from it. When she is nearest, she attracts strongest,and so raises the Tides most; the contrary happens when she is farthest, because of her weaker attraction.

When both luminaries are in the equator, and the Moon in perigee, (or at her least distance from the earth,) she raises the Tides highest of all; especially at her conjunction and opposition, both because the equatorial parts have the greatest centrifugal force from their describing the largest circle, and from the concurring actions of the Sun and Moon. At the change, the attractive forces of the Sun and Moon being united, they diminish the gravity of the waters under the Moon, and their gravity on the opposite side is diminished by means of a greater centrifugal force.— At the full, while the Moon raises the Tide under, and opposite to her; the Sun acting in the same line, raises the Tide under, and opposite to him; whence their conjoint effect is the same as at the change, and in both cases, occasion what is called, Spring Tides. But at the quarters, the Sun's action diminishes the action of the Moon on the waters, so that they rise a little under, and opposite to the Sun, and full as much under, and opposite to the Moon, making what we call Neap Tides; because the Sun and Moon then act crosswise to each other. But strictly speaking, these Tides happen not till some time after, because in this, as in other cases, the actions do not produce the greatest effect, when they are at the strongest, but sometime afterward.-The Sun being nearer the earth in Winter than in Summer, is of course, nearer to it in February and October, than in March and September, and therefore the greatest Tides happen not till some time after the autumnal equinox and return a little before the ver