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them of the light of the moon. At first they were indifferent to his threats, but "when the eclipse actually commenced, the barbarians vied with each other in the production of the necessary supplies for the Spanish fleet."
Description.—Twice a day, at intervals of about twelve hours and twenty-five minutes, the water begins to set in from the ocean, beating the pebbles and the foot of the rocky shore, and dashing its spray high in air. For about six hours it climbs far up on the beach, flooding the low lands and transforming simple creeks into respectable rivers. The instant of high-water or flood-tide being reached, it begins to descend, and the ebb succeeds the flow. The water, however, falls somewhat slower than it rose.
Cause Of The Tides.—The tides are caused by a great wave, which, raised by the moon's attraction,
follows her in her course around the earth. The sun, also, aids somewhat in producing this effect; but asthe moon is 400 times nearer the earth, her influence is far greater. As the waters are free to yield to the attraction of the moon, she draws them away from C and D and they become heaped up at A. The earth, being nearer the moon than the waters on the opposite side, is more strongly attracted, and so, being drawn away from them, they are left heaped up at B. As the result, high-water is produced at A by the water being pulled from the earth, and at B by the earth being pulled from the water. The influence of the moon is not instantaneous, but requires a little time to produce its full effect; hence high-water does not occur at any place when the moon is on the meridian, but a few hours after. As the moon rises about fifty minutes later each day, there is a corresponding difference in the time of high-water. While, however, the lunar tidewave thus lags about fifty minutes every day, the solar tide occurs uniformly at the same time. They therefore steadily separate from each other. At one time they coincide, and high-water is the sum of lunar and solar tides; at other times, high-water of the solar tide and low-water of the lunar tide occur simultaneously, and high-water is the difference between the lunar and solar tides.
We should bear in mind the philosophical truth, that the tide in the open sea does not consist of a progressive movement of the water itself, but only of the form of the wave.
Causes that modify the tides.—At new and full moon (the syzygies) the sun acts with the moon (as in Fig. 53) in elevating the waters; this produces the highest or Spring tide. In quadrature (as in Fig. 51), the sun tends to diminish the height of the water: this is called Neap-tide. When the moon is in perigee her attraction is stronger; hence the flood-tide is higher and the ebb-tide lower than at other times. This re
mark applies also to the sun. The height of the tide also varies with the declination of the sun and moon, —the highest or equinoctial tides taking place at the equinoxes, if, when the sun is over the equator, the moon also happens to be very near it: the lowest occur at the solstices. The force and direction of the winds, the shape of the coast, and the depth of the sea wonderfully complicate the explanation of local tides.
Height of the tide at different places.—In the open sea the tide is hardly noticeable, the water sometimes rising not higher than a foot; but where the wave breaks on the shore, or is forced up into bays or narrow channels, it is very conspicuous. The difference between ebb and flood neap-tide at New York is over three feet, and that of spring tide over
five feet; while at Boston it is nearly double this amount. A headland jutting out into the ocean will diminish the tide; as, for instance, off Cape Florida, where the average height is only one and a half feet. A deep bay opening up into the land like a funnel, will converge the wave, as at the Bay of Fundy; where it rolls in, a great roaring wall of water sixty feet high, frequently overtaking and sweeping off men and animals. The tide sets up against the current of rivers, and often entirely changes their character; for example, the Avon at Bristol is a mere shallow ditch, but at flood-tide it becomes a deep channel navigable by the largest Indiamen.
Differential effect.—The whole attraction of the moon is only that of the sun: yet her influence in producing the tides and precession is greater, because that depends not upon the entire attraction either exerts, but upon the difference between their attraction upon the earth's centre and upon the earth's nearest surface. For the moon, on account of her nearness, the proportion of the distance of these parts is treble that of the sun, and hence her greater effect.
Description.—Passing outward in our survey of the solar system, we next meet with Mars. This is the first of the superior planets, and the one most like the earth. It appears to the naked eye as a bright
red star, rarely scintillating, and shining -with a steady light, which distinguishes it from the fixed stars. Its ruddy appearance has led to its being celebrated among all nations. The Jews gave it the appellation of "blazing," and it bore in other languages a similar name. At conjunction its apparent
diameter is only about 4"; but once in two years it comes into opposition with the sun, when its diameter increases to 30". At intervals of 8yr. 7 mo. this occurs when the planet is also in perihelion and perigee. Mars then shines with a brilliancy rivalling that of Jupiter himself.
Motion In Space.—Mars revolves about the Sun at a mean distance of about 140,000,000 miles. Its orbit is sufficiently flattened to bring it at perihelion 26,000,000 miles nearer that luminary than when in aphelion. Its motion varies in different portions of its orbit, but the average velocity is about fifteen