C. It was therefore concluded that light is not instantaneous, but requires a certain space of time to pass from one region of the universe to another, and that the time it takes in passing from the sun to the earth, or across the semidiameter of the earth's orbit, is eight minutes and a quarter, or at the rate of 192,000 miles every second, which is more than ten hundred thousand times swifter than a cannon ball the moment it is projected from the mouth of the cannon; and therefore it is the swiftest movement with which we are acquainted in nature. It follows that, if the sun was annihilated, we should see him for eight minutes afterward; and if he were again created, it would be eight minutes before his light would be perceived. The motion of light deduced from the eclipses of Jupiter's satellites has been confirmed by Dr. Bradley's discovery of the aberration of light produced by the annual motion of the earth, from which it appears that the light from the fixed stars moves with about the same velocity as the light of the sun.

III. ON THE SATELLITES OF SATURN.

Saturn is surrounded with no less than seven satellites, which revolve around him, at different distances, in a manner similar to those of Jupiter. As they are more difficult to be perceived than the satellites of Jupiter, owing to the great distance of Saturn from the earth, none of them were discovered till the telescope was considerably improved; and more than a century intervened after the first five satellites till the sixth and seventh were detected. As was to be supposed, the larger satellites were first discovered. In the year 1665, about forty-five years after the invention of the telescope, M. Huygens, a celebrated Dutch mathematician and astronomer, discovered the fourth satellite, which is the largest, with a telescope twelve feet long. Four of the others were discovered by Cassini; the fifth in 1671, which is next in brightness to the fourth; the third in December, 1672; and the first and second in the month of March, 1684. These four satellites were first observed by common refracting telescopes of 100 and 136 feet in length; but, after being acquainted with them, he could see them all, in a clear sky, with a tube of thirty-four feet. The sixth and seventh satellites, were discovered by Sir W. Herschel in August, 1789, soon after his large forty feet reflecting telescope was completed. These are nearer to Saturn than the other five; but, to avoid confusion, they are named in the order of their discovery. The following is the order of the satellites in respect of their distance from Sa

The motions and distances of these bodies have not been so accurately ascertained as those of Jupiter. The following statement contains a near approximation of their periods and distances. The seventh satellite, or that nearest to Saturn, is distant, 120,000 miles from the centre of the planet, about 80,000 from its surface, and only about 18,000 miles beyond the edge of the outer ring. It moves round the planet in twenty-two hours, thirtyseven minutes, a circuit of 377,000 miles, at the rate of 16,755 miles an hour. The sixth satellite, or the second from Saturn, is distant 150,000 miles, and finishes its revolution in one day, eight hours, fifty-three minutes. The first of the old satellites, or the third from Saturn, finishes its periodical revolution in one day, twenty-one hours, eighteen minutes, at the distance of 190,000 miles. The second (or fourth from Saturn,) in two days, seventeen hours, forty-four and three quarter minutes, at the distance of 243,000 miles. The third (fifth from Saturn,) in four days, twelve hours, fifty-five minutes, at the distance of 340,000 miles. The fourth (sixth from Saturn,) in fifteen days, twenty-two hours fifty-one minutes, at the distance of 788,000 miles. The fifth (seventh from Saturn,) in seventy-nine days, seven hours, and fifty-four and a half minutes, at the distance of 2,297,000 miles.

The orbits of the six inner satellites are inclined about thirty degrees to the plane of Saturn's orbit, and lie almost exactly in the plane of the rings, and therefore they appear to move in ellipses similar to the ellipses of the rings. But the orbit of the fifth or outer satellite, makes an angle with the plane of Saturn's orbit of 24 degrees, 45 minutes. These satel lites, having their orbits inclined at so great angles to Saturn, cannot cross the body of that planet, or go behind it, or pass through its shadow, as Jupiter's satellites do, except on rare occasions, and hence they very seldom suffer eclipses or occultations. The only time when eclipses happen is near the periods when the ring is seen edgewise. The fifth or most distant satellite is sometimes invisible in the eastern part of its orbit, which is supposed to arise from one part of the satellite being less luminous than the rest. Sir W. Herschel observed this satellite through all the variations of its light, and concluded, as Cassini had done before, that it turned round its axis like our moon, in the same time that it performed its revolution round Saturn. In consequence of this rotation, the obscure part of its disk is turned towards the earth when in the part of its orbit east of Saturn; and the luminous portion of its surface is turned to the earth and becomes visible while it passes through the western part of its course.

Of these satellites the two innermost are the smallest and the most difficult to be perceived. They have never been discerned but with the most powerful telescopes, and then under peculiar circumstances. At the time of the disappeararice of the ring, "they have been seen threading, like beads, the most infinitely thin fibre of light to which it is then reduced, and, for a short time, advancing off it at either end." Few astronomers besides Sir W. Herschel and his son have been able to detect these small bodies. The celebrated Schroeter and Dr. Harding, on the 17th, 20th, 21st, and 27th of February, 1798, obtained several views of the sixth satellite (the second from Saturn) by means of a reflecting telescope 13 feet long, carrying a power of 288. Their observations fully confirmed the accuracy of Sir W. Her schel's statement of the period of its revolution. The first and second satellites (third and fourth from Saturn) are the next smallest ; the third (fifth from Saturn) is greater than the first and second; the fourth (sixth from Saturn) the most conspicuous and the most distant satellite, according to Sir John Herschel, is by far the largest, although it is not so conspicuous in one part of its orbit. In order to see any of the satellites of this planet, a good telescope, with a power of at least 70 or 80 times, is requisite, and with such a power only the two outermost satellites will be perceived. To perceive all the five old satellites requires a power of at least 200 times, and a considerable quantity of light.

Magnitude of Saturn's Satellites.-The precise bulk of these satellites has not yet been accurately determined. Sir John Herschel estimates the most distant satellite, which he thinks the largest, as not much inferior in size to the planet Mars, which is 4200 miles in diameter. The fourth satellite, which is the most conspicuous, cannot be supposed to be much inferior to it in bulk. But as the precise dimensions of most of the inner satellites cannot be estimated with accuracy, we shall not, perhaps, exceed the dimensions of these Dodies if we suppose for the whole a general average of 3000 miles diameter for each. On this assumption, the surface of each satellite will contain 28,274,400 of square miles, which is nearly double the area of our moon. The area of all the seven satellites will therefore amount to 197,920,800 square miles, which is four times the quantity of surface on all the habitable parts of the earth. At the rate of 280 inhabitants to the square mile, these satellites would therefore contain 55,417,824,000, or more than fifty-five thousand millions of inhabitants, which is sixty-nine times the population of our globe.

These satellites will present a beautiful and variegated appearance in the firmament of Sa

turn; the nearest satellite, being only 80,000 miles from the surface of the planet, which is only the one third of the distance of the moon from the earth, will exhibit a very large and splendid appearance. Supposing it to be only about the diameter of our moon, it will present a surface nearly nine times larger than the moon does to us; and in the course of twentytwo and a half hours will exhibit all the phases of a crescent, half moon, full moon, &c., which the moon presents to us in the course of a month; so that almost every hour its phase will be sensibly changed, and its motion round the heavens will appear exceedingly rapid. While, in consequence of the diurnal rotation of Saturn, it will appear to move from east to west, it will also be seen moving with a rapid velocity among the stars in contrary direc tion, and will pass over a whole hemisphere of the heavens in the course of eleven hours. The next satellite in order from Saturn, being only 110,000 miles from his surface, will also present a splendid appearance, much larger than our moon, and will exhibit all the phases of the moon in the course of sixteen hours. All the other satellites will exhibit somewhat similar phenomena, but in different periods of time. They will appear, when viewed from the surface of Saturn, of different sizes; some of them nine times larger than the moon appears to us, some three times, some double the size, and it is probable that even the most distant satellites will appear nearly as large as our moon, so that a most beautiful and sublime variety of celestial phenomena will be presented to a spectator in the heavens of Saturn, besides the diversified aspects of the rings to which we formerly adverted, all displaying the infinite grandeur and beneficence of the Creator.

IV. ON THE SATELLITES OF URANUS.

This planet is attended by six satellites, all of which were discovered by Sir W. Herschel, to whom we owe the discovery of the planet itself. The second and fourth satellites were detected in January, 1787, about six years after the planet was discovered; the other four were discovered several years afterward, but their distances and periodical revolutions have not been so accurately ascertained as those of the two first discovered.

The first of these satellites, or the nearest to Uranus, completes its sidereal revolution in 5 days, 21 hours, and 25 minutes, at the distance of 224,000 miles from the centre of the planet. The second in 8 days, 17 hours, at the distance of 291,000 miles. The third in 10 days, 23 hours, at the distance of 340,000 miles. The fourth in 13 days, 11 hours, at the distance of 390,000 miles. The fifth in 38 days, one hour, 48 minutes, at the distance

of 777,000 miles. The sixth in 107 days, 16 hours, 40 minutes, at the distance of 1,556,000 miles.

These bodies present to our view some remarkable and unexpected peculiarities. Contrary to the analogy of the whole planetary system, the planes of their orbits are nearly perpendicular to the ecliptic, being inclined no less than 79 degrees to that plane. Their motions in these orbits are likewise found to be retrograde, so that, instead of advancing from west to east round Uranus, as all the other planets and satellites do, they move in the opposite direction. Their orbits are quite circular, or very nearly so, and they do not appear to have undergone any material change of inclination since the period of their discovery. "These anomalous peculiarities," says Sir John Herschel, "seem to occur at the extreme limits of the system, as if to prepare us for further departure from all its analogies in other systems which may yet be disclosed to us" in the remoter regions of space.

The satellites of Uranus are the most difficult objects to perceive of any within the boundary of the planetary system, excepting the two interior satellites of Saturn; and therefore few observers, excepting Sir William and Sir John Herschel, have obtained a view of them. Their magnitudes, of course, have never been precisely determined; but there is every reason to believe that they are, on an average, as large as the satellites of Saturn, if not larger, otherwise they could not be perceived at the immense distance at which they are placed from our globe. Supposing them, on an average, to be 3000 miles in diameterand they can scarcely be conceived to be less -the surfaces of all the six satellites will contain 169,646,400 square miles, or about 3 times the area of all the habitable portions of the earth; and which, at the rate formerly stated, would afford scope for a population of 47,500,992,000, or above forty-seven thousand millions, which is about sixty times the present number of the inhabitants of the earth.

The satellites of Uranus seldom suffer eclipses; but as the plane in which they move must pass twice in the year through the sun, there may be eclipses of them at those times; but they can be seen only when the planet is near its opposition. Some eclipses were visible in 1799 and 1818, when they appeared to

ascend through the shadow of the planet in a direction almost perpendicular to the plane of its orbit. It is probable that this planet is attended with more satellites than those which have yet been discovered. It is not unlikely that two satellites at least revolve between the body of the planet and the first satellite; for the third satellite of Saturn is not nearly so far distant from the surface of that planet as the first satellite of Uranus is from its centre. But as the inner satellites may be supposed to be the smallest, and yet present as large a surface to the planet as the exterior ones, it is probable that, on account of their diminutive size, they may never be detected. It is likewise not improbable that two satellites may exist in the large spaces which intervene between the orbits of the fourth and fifth, and the fifth and sixth satellites. All these satellites will not only pour a flood of light on this distant planet, but will exhibit a splendid and variegated appearance in its nocturnal firmament.

The satellites of Jupiter, Saturn, and Uranus, of which we have given a brief description in the preceding pages, form, as it were, so many distinct planetary systems in connexion with the great system of the sun. The same laws of motion and gravitation which apply to the primary planets are also applicable to the secondary planets or moons. The squares of their periodical times are in proportion to the cubes of their distances. They are subject to the attraction of their primaries, as all the primary planets are attracted by the sun; and as the sun, in all probability, is carried round a distant centre along with all his attendants, so the satellites are carried round the sun along with their respective planets; partly by the influence of these planets, and partly by the attractive power of the great central luminary. Each of these secondary systems forms a system by itself, far more grand and extensive than the whole planetary system was conceived to be in former times. Even the system of Saturn itself, including its rings and satellites, contains a mass of matter more than a thousand times larger than the earth and moon. The system of Jupiter comprises a mass of matter nearly fifteen hundred times the size of these two bodies; and even that of Uranus is more than eighty times the dimensions of our terrestrial system.