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ter with our antipodes at the same time that it is high water with us.
At any port or harbour lying open to the ocean, the moon's attractive power tends to elevate the waters when she is on the meridian of that place, whether it be above or below the horizon. The water, however, cannot be elevated at one place without being depressed at another, whence it has been drawn; and it is sufficiently evident that these elevations and depressions must be most considerable at opposite points of the earth's surface.
When the moon raises the waters at one of these points, they will, of course, be depressed at the other and as the moon passes over the meridian and is in the horizon twice every day, there must consequently be two ebb and two flood tides in that time, at the interval of six hours and eleven minutes each.
From a cursory perusal of the above remarks, our young readers might conjecture that the time of high water at any particular place would be when the moon was over the meridian of that place, whereas it usually happens about three hours afterward.
To account for this we observe, that the moon, when on the meridian most contiguous to the zenith of any place, tends to elevate the waters at that place; but this force must be exerted for a considerable space of time before the water attain its utmost elevation; for if the attraction of the moon were to cease altogether after she had passed
the meridian, yet from the motion previously communicated to the waters, they would con⚫ tinue to ascend for some time afterwards consequently they must be still more disposed to ascend when the attractive force has suffered only a small diminution. These effects, like the continuation of the waves of the sea after a storm has subsided, and the increase of heat two or three hours after noon on the longest day, may be easily understood, from the consideration that a small impulse given to a body in motion will cause it to move further than it would have done without such impulse. Instead of its being high water, therefore, when the moon is upon the meridian of any place, it will be found to happen about three hours afterward.
It is necessary to observe that tides might be produced by the influence of the sun, though, as we have already observed, in a much less degree. The whole attractive force of the sun indeed, is greatly superior to that of the moon, but as the distance from our globe is nearly four hundred times greater, the forces with which he acts upon different parts of it are much more equal than those of the moon, and, of course, have less effect in occasioning any mutation of its figure.
In illustration of this doctrine it may be observed, that notwithstanding the considerable proportion which the earth's diameter bears to its distance from the moon, yet this diameter is scarcely any thing in comparison
with the earth's distance from the sun. The difference, therefore, of the sun's attraction on those sides of the earth which lie under and opposite to him, will be considerably less than the difference of the moon's attraction on the sides of the earth which are under and opposite to her; consequently the moon must elevate the tides much higher than they can be raised by the sun. And as the great Sir Isaac Newton discovered the effect of the sun's influence in this case to be three times less than that of the moon, it is evident that although the sun alone might occasion an ebbing and flowing of the sea, the elevation and depres sion thus produced would be three times less than those occasioned by the moon.
From these considerations it may be readily perceived that the tides are produced by the joint forces of the sun and moon; or, more strictly speaking, there are a solar and a lunar tide, which have an opposite, or a combined effect, according to the situation of the bodies by which they are produced.
When the actions of the sun and moon are united, as at the time of new and full moon, the ebbing and flowing of the sea become more considerable, and are, therefore, called the Spring tides; but when one of those luminaries tends to raise the waters while the other depresses them, as in the first and third quarters of the moon, the ebbing and flowing will be diminished, and are then called Neap tides. It is necessary, however, to remark, that nei
ther of these tides happens when the sun and moon are precisely in the situations alluded to: because their actions do not produce the greatest effect when they are the strongest, but some time afterwards, as we have previously intrmated.
It must, also, be observed, that the agitating forces of the sun and moon depend not only upon their particular situations, but, also, upon their respective distances from the earth: for their effects will be greater in proportion to their contiguity; whence it happens that the spring tides are greatest in winter, when the sun is farthest off, and the neap tides are consequently least. For a similar reason, as the moon travels round our globe in an elliptic orbit, and is nearer at some times than at others, the tides, at such times are invariably found to be augmented or depressed.
Some variations are, likewise, produced by the different declinations of the two great luminaries; for if either the sun or moon were at one of the poles, it would occasion a constant swell both there and at the opposite pole, and a constant ebb, or depression about the equator: in proportion, therefore, as these tuminaries decline from the equator, they gradually lose their effect, and the tides are diminished; but when both of them are in the equator, the tides are consequently augmented.
In open seas the elevation of the tides is very inconsiderable, in comparison of that which is observed in wide-mouthed rivers
opening in the direction of the stream of tide: for in channels which gradually become narrower, the water is accumulated by the opposition of the contracting banks; in the same manner as a gentle wind, though scarcely felt on an extensive plain, seems brisk in a street; particularly if the widest end of the street open to the plain, and in the direction of the wind.
Tides in respect of different places, happen at all distances of the moon from the meridian, owing to their being obstructed in their passage through different shoals and channels, and being also variously affected by striking against capes, headlands, &c. consequently they occur at all hours of the day. The lunar tide produced in the German ocean when the moon is three hours past the meridian, takes twelve hours to flow from thence to London-bridge, where it arrives by the time that a new tide is raised in the ocean. At many places, also, it is high water three hours before the moon comes to the meridian; but that tide which the moon seems to push before her is merely the tide opposite to that which she raised when nine hours past the opposite meridian.
Lakes do not exhibit the phenomenon of tides; for they are, in general, so small, that when the moon is vertical she attracts every part of the water equally, and consequently it cannot be elevated in one place and depressed in another. The Baltic and Mediter ranean seas have very trifling elevation; the