of the earth's shadow, reduce the remainder, and also the Moon's latitude to seconds, find their sum and difference, multiply the sum by the difference, and the square root of the product will be the scruples of total darkness; then as the hourly motion of the Moon from the Sun is to one hour, so are the scruples of total darkness to the time of its half duration. To the middle of the eclipse add the time of half duration, and the sum will be the time when total darkness ends. Subtract it, and the remainder will show the time that total darkness

commences.

It is difficult to observe exactly, either the beginning or ending of a lunar eclipse, even with a good telescope; because the earth's shadow is so faint and ill-defined about the edges, that when the Moon is either just touching or leaving it, the obscuration of her limb is scarcely sensible, and therefore the closest observers can hardly be certain to four or five seconds of time.

But both the beginning and ending of solar eclipses are instantaneously visible; for the moment that the edge of the Moon's disk touches the Sun's, his roundness appears to be broken on that part, and the moment she leaves it, he appears to be round again.

In Astronomy, eclipses of the Moon are of great use in ascertaining the periods of her motions, especially such eclipses as are observed to be alike in all circumstances, and have long intervals of time between them. In Geography, the longitudes of places are found by eclipses. The eclipses of the Moon are more useful for this purpose than those of the Sun, because they are more frequently visible, and the same lunar eclipse is equally large at all places where it is seen.

In Chronology, both solar and lunar eclipses serve to determine exactly the time of any past event; for there are so many particulars observable in every eclipse with respect to its quantity, the places where it is perceivable, (if of the Sun,) and the time of the day or night, that it is impossible that there can be two solar eclipses in the course of many ages, which are alike in all circum

SECTION EIGHTEENTH.

ON THE FIXED STARS.

THE Stars are said to be fixed, because they have been generally observed to keep at the same distances from each other; their apparent diurnal revolutions being caused solely by the earth's turning on its axis. They appear of a sensible magnitude to the eye, because the retina is affected not only by the rays of light which are emitted directly from them, but by many thousands more, which, falling upon our eyelids and upon the aerial particles about us, are reflected into our eyes so strongly as to excite vibrations not only in those points of the retina where the real images of the stars are formed, but also in other points of some distance round. This makes us imagine the stars to be much larger than they would appear if we saw them only by the few rays which come directly from them, so as to enter our eyes, without being intermixed with others. Any person may be sensible of this, by looking at a star of the first magnitude, through a long, narrow tube, which, though it takes in as much of the sky as would hold a thousand such stars, yet scarcely renders that one visible.

The more a telescope magnifies, the less is the aperture through which the star is seen; and consequently the less number of rays it admits into the eye. The stars appear less in a telescope which magnifies two hundred times, than they do to the naked eye; insomuch that they seem to be only indivisible points: it proves at once that the stars are at immense distances from us, and that they shine by their own proper light. If they shone by reflection, they would be as invisible without telescopes as the satellites of Jupiter. These satellites appear larger when viewed with a good telescope, than any of the fixed stars.

The number of stars discoverable in either hemisphere by the unaided sight, is not above a thousand. This at first may appear incredible, because they seem to be almost innumerable; but the deception arises from our looking confusedly upon them, without reducing them to any order: look steadfastly upon a large portion of the sky, and count the number of stars in it, and you will be surprised to find them so few. Consider only how seldom the Moon passes between us and any star, (although there are as many about her path as in any other parts of the heavens,) and you will soon be convinced that the stars are much thinner sown than you expected. The British catalogue, which, besides the stars visible to the naked eye, includes a great number which cannot be seen without the assistance of a telescope, contains no more than three thousand in both hemispheres.

As we have incomparably more light from the Moon than from all the stars together, it is the greatest absurdity to imagine that the stars were made for no other pur pose than to cast a faint light upon the earth, especially since many more require the assistance of a good telescope to find them out, than are visible without that instrument. Our Sun is surrounded by a system of planets and comets, all of which would be invisible from the nearest fixed star. And from what we already know of the immense distance of the stars, the nearest may be computed at thirty-two billions of miles from us, which is farther than a cannon ball can fly in seven millions of years, though it proceeded with the same velocity as at its first discharge. Hence it is easy to prove, that the Sun, seen from such a distance, would appear no larger than a star of the first magnitude. From the foregoing observations, it is highly probable, that each star is the centre of a magnificent system of worlds, moving round it, though unseen by us, and are irradiated by its beams: especially as the doctrine of plurality of worlds is rational, and greatly manifests the power, wisdom, and goodness of the great Creator.

The stars, on account of their apparently various magnitudes, have been distributed into several classes or orders. Those which appear largest, are called stars of the first magnitude; the next to them in lustre, stars of the second magnitude; and so on to the sixth, which are the smallest that are visible to the unaided sight. This distribution having been made long before the invention of telescopes, the stars which cannot be seen without the assistance of these instruments, are distin guished by the name of telescopic stars.

The ancients divided the starry spheres into particular constellations, or systems of stars, according as they lay near each other, so as to occupy those spaces which the figures of different sorts of animals or things would take up, if they were there delineated. And those stars which could not be brought into any particular constellation, were called unformed stars.

a

The division into different constellations or asterisms, serves to distinguish them from each other; so that any particular star may be readily found in the heavens, by means of a celestial globe, on which the constellations are so delineated as to put the most remarkable stars into such parts of the figures as are most easily distinguished. The number of ancient constellations is 46, and upon our present globes about 70. There is also division of the heavens into three parts. First, the zodiac, signifying an animal, because most of the constellations in it, which are twelve in number, are the figures of animals, as Aries, the ram; Taurus, the bull; Gemini, the twins; Cancer, the crab; Leo, the lion; Virgo, the virgin; Libra, the balance; Scorpio, the scorpion; Sagitarius, the archer; Capricornus, the goat; Aquarius, the water-bearer; and Pisces, the fishes. The zodiac goes quite round the heavens; it is about sixteen degrees broad, so that it takes in the orbits of the Moon, and of all the planets, (excepting that of Pallas, and the satellites of Herschel.)