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digits, according to the number of those parts which are obscured. It is also necessary to remark, that an eclipse of the sun always begins on his western side and goes off on his eastern; and that an eclipse of the moon begins on her eastern side, and goes off on her western. And all that the moon is eclipsed adove twelve digits, marks how far the earth's shadow extends over her body, on that edge to which she is nearest at the middle of the eclipse.
Solar eclipses are more frequent than lunar ones, because the sun's ecliptic limits are greater. There are, however, more visible eclipses of the moon than of the sun, because these are seen from all parts of the earth where the moon is above the horizon at the time the eclipse happens, whilst those of the sun can only be observed on that small portion of the hemisphere in which the moon's shadow falls.
The greatest number of eclipses of both luminaries which can happen in one year is seven, and the least two; but the most usual number is four; and there are scarcely ever more than six, of which one half are generally invisible at any particular place.
From a due consideration of these particu lars, the young reader will comprehend the doctrine of eclipses, and will perceive that it is no difficult matter for astronomers to predict the exact time when any phenomenon of this kind will make its appearance. For as
an eclipse can only happen at the time of new or full moon, the principal requisites are to ascertain the number of mean conjunctions and oppositions that will occur every year, and the exact situations of the sun and moon in their orbits at each of these times. And if it then appear, that the two luminaries are within the proper limits of the node, there will be an eclipse, or otherwise not, agreeably to our previous observations.
It is extremely difficult, even with a good telescope, to observe exactly either the com mencement or ending of a lunar eclipse; for the earth's shadow is so faint and imperfectly defined about the edges, that when the moon is either just touching or leaving it, the obscu ration of her limb is scarcely sensible; and consequently the most attentive observer can hardly deem himself certain within several seconds. The beginning and ending of solar eclipses, however, are visibly instantaneous; for at the moment that the edge of the moon's disk touches that of the sun, his roundness seems rather broken on that part; and the instant she leaves it, he appears perfectly round again.
The utility of eclipses is equally great and important in the sciences of astronomy, geography, and chronology. In astronomy, for instance, eclipses of the moon are of the ut most use for ascertaining the periods of her motions, particularly such eclipses as are alike in all circumstances, and have long interval
of time between them: in geography the longitude of any particular place may be found by eclipses, as we have explained in a former chapter; and in chronology both solar and lunar eclipses serve to fix the precise time of any past event; for so many particulars are observable in every eclipse (especially of the sun,) with respect to its quantity, the places where it is visible, and the time of the day or night, that there cannot be two solar eclipses similar in all circumstances in the course of many ages.
Of the Phenomena of the Tides, Winds, &er
IT is a known fact that more than half the surface of the earth is covered with water, and this large fluid body appears in continual motion, ebbing and flowing alternately without any intermission. It is also worthy of observation, that these motions invariably follow one certain rule; for if the tide at a given hour, be at high-water mark in any port or harbour lying open to the ocean, it will soon begin to sink, and flow regularly back till it be found at low water-mark, which will be in about six hours, After this it will gradually 7 VOL, I.
rise for six hours, and then subside in the same time ebbing and flowing by turns twice in the space of twenty-four hours.
The interval between the rising and falling of the tide, however, is not exactly six hours, but about eleven minutes more, so that the time of high water happens about forty-five minutes later every day for thirty days; after which it recurs as before. Thus if it be high water to-day at noon, it will be low water at eleven minutes past six in the evening, and after two more changes, the high water next day will happen about three quarters of an hour past twelve; the day following it will occur at half an hour past one, and so on for thirty days, when it will be high water at noon, as on the day when the observation was first made. This corresponds exactly with the motion of the moon; for she rises every day about forty-five minutes later than on the preceding one, and by moving in this manner round our globe, completes her revolution in about thirty days; after which she begins to rise again at the same time as before.
A harmony of motions so perfect and exact could not possibly result from a mere concurrence of fortuitous causes; but independently of all mathematical considerations we must be induced to look to the moon as their cause. It therefore remains to show that these phenomena are principally occasioned by the moon's attraction: for the sun's power in raising the
tides must be comparatively small on account of his immense distance from the earth.
The attractive force of the moon acts unequally upon different parts of the ocean, and consequently causes it to assume a different figure from what it would otherwise have. And as this attractive force is most powerful on that part of the ocean immediately under the moon, the waters will, therefore, flow constantly to that part, and be higher or lower at different places according as her situation varies with respect to those places. But as the earth turns on its axis, in about twentyfour hours and three quarters, from the moon to the moon again the flux and the reflux will, of course, be retarded from day to day about forty-five minutes.
Hence it is sufficiently obvious that the tides are occasioned by the attractive force of the moon; but we have yet to explain the more singular circumstance of their ebbing and flowing twice every twenty-four hours.
When the moon passes the meridian, or is at her greatest height above the horizon of any place, she will naturally attract and elevate the waters lying immediately under her; but it may be asked why the waters are elevated at the same place twelve hours afterward, when the moon passes the meridian below the horizon? for it is a known fact, that whether she be in the zenith or nadir the phenomenon is nearly the same, it being high was