OF TUE TIDES.
The ebbing and flowing of the sea was first shown by Kepler to be owing to the moon's attraction, and Newton demon strated it upon the principles of gravita tion. The attraction of the moon cannot alter the shape of the solid of the globe ; but it has a considerable effect upon the fluid part, which it causes to assume a spheroidal figure, the longest axis being in the direction of the moon. It is there fore the highest tide at that place per pendicularly under the moon, or where the moon crosses the meridian. The sun also has some action upon the waters, though its attraction, on account of its dis tance, is not so strong as that of the moon. When the action of the sun and moon conspire together, the tide rises higher, and produces what are called spring tides. On the contrary, when they counteract each other, they produce neap tides. The ocean, it is well known, covers more than one half of the globe ; and this large body of water is found to be in continual mo tion, ebbing and flowing alternately, with out the least intermission. What con nection these motions have with the moon we shall see as we proceed ; but at present it will be sufficient to observe, that they always follow a certain general rule. For instance, if the tide be now at high-water mark in any port or harbour which lies open to the ocean, it will pre sently subside, and flow regularly back for about six hours, when it will be found at low-water mark. After this, it will again gradually advance for six hours, and then return back in the same time to its former situation ; rising and falling, alternately, twice a day, or in the space of twenty-four hours. And by observing the tides continually at the same place, they will always be found to follow the same rule ; the time of high water upon the day of every new moon being nearly at the same hour, and three-quarters of an hour later every succeeding day. Let 11 (fig. 3.) represent the moon, 0 the centre of the earth, and A, B, C, &c. different points upon its surface, and let us suppose the earth to be entirely co vered by the ocean. Then, because it is the property of a fluid for its parts to yield, and obey any force impressed upon them, it is clear that the moon M acting upon the surface of the sea at the points A, B, C, &c. will elevate the waters in those
parts, and draw them towards her, by her Attractive power. But the point A being nearer to the moon than the point C, the attraction at A will be greater than at C and because the points B and D are at equal distances from the moon, the at traction at those points will also be equal; and so at any other intermediate points the attractive force will be different, ac cording to their different distances from the moon.
From this example, then, it is sufficient ly evident, that the attractive force of the moon, acting unequally upon different parts of the ocean, must occasion it to assume a different figure from what it would otherwise have, if there were no such unequal attractions. And since this attractive force is greatest on the part of the ocean which lies immediately under the moon, the waters will of course flow constantly to that part, and he elevated or depressed at different places, according as her situation changes with respect to those places. But, as the earth turns round on its axis, from the moon to the moon again, in about twenty-four hours and three-quarters, the flux and re flux will be necessarily retarded from day to day about three quarters of an hour, which is agreeable to experience. It re mains now to be explained, why they ebb and flow twice a day, or in the space of about twenty-four hours. When the moon passes the meridian of any place, or is at her greatest height above the horizon of that place, she will evidently attract and elevate the waters which lie immediately under her : but what is the reason, that, twelve hours afterwards, when she passes the meridian below the horizon, the waters at the same place are then also elevated ? We know, from ex perience, that, whether the moon be in the zenith or nadir, the phenomenon is nearly the same ; it being high water with us at the same time that it is high water with our antipodes.