TIDE. The regular periodical current of water, which, when it rises, is called the flux, and when it goes back is the ebb or reflux. The and flowing of the sea was first by Kepler to be owing to the moon's attraction ; and Newton demonstrated it upon the principles of gravitation. The attraction of the moon cannot alter the shape of the solid part of the globe, but it has a considerable effect upon the fluid part, which it causes to assume a spheroidal figure, the longer axis being in the direction of the moon. It is, there fore, the highest tide at that place perpendicu larly under the moon, or where the moon crosses the meridian. The sun also has some action upon the waters, though its at traction, on account of its distance, is not ss 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 a large body of wa ter is found to be in continual motion, ebbing and flowing alternately, without intermission.
What connexion these motions have with the moon we cannot here detail ; in this place it will be sufficient to observe, that they always follow a Certain general rule. For instance, it the tide be now at high water mark in any port or harbour which lies open to the ocean, A will presently subside, and flow regularly back for about six hours, when it willbe found at low water mark. After this, it will again gradually advance for six hours, and then re turn back in the same time to its former situa tion: rising and falling alternately twice a day, or in the space of about twenty-four hours. And by observing the tides continually at the same place, they will always be found to fol low the same rule; the time of high water upon the day of every new moon being nearly at the same hour, andthree quarters of an hour later every succeeding day.