URANUS.-NEPTUNE. are only certain of four, two of which were observed by Herschel. The remaining two are nearer the planet than the innermost of Herschel's. These four satellites are known by the names of Ariel, Umbriel, Titania, and Oberon. The diameter of Uranus is about 33,000 miles, and its volume or bulk is seventyfour times greater than that of the Earth. The solar light and heat reflected on the surface of a planet so distant as Uranus must be very small comparatively to what we are receiving, if a constant absorption of the light and heat has taken place while passing through space. Assuming the solar influence on the Earth to equal 1, that at Uranus, which is more than nineteen times farther from the Sun, amounts only to 0.003, or the three-thousandth part of that to which we are accustomed. NEPTUNE. The history of the discovery of Neptune is a record of one of the greatest triumphs of intellectual skill which have graced the progress of astronomical knowledge from the earliest ages to the present time. Two mathematicians, working independently of each other, each believing faithfully in the truth of the laws of universal gravitation, and having for the subject of their investigations certain observed irregularities in the motion of Uranus in its orbit, which could only be accounted for by supposing them caused by the attraction of some unknown large planet at a still greater distance from the Sun, announced to the practical astronomer, that if the adopted theory of the universe be true, the supposed body must be in a certain position of the heavens. These two mathematicians, M. Le Verrier, of Paris, and Mr. J. C. Adams, of Cambridge, sent the results of their calculations to the principal astronomers, some of whom organised a series of observations for a systematic search. When Dr. Galle, of Berlin, received on September 23rd, 1846, the indication of the place of the supposed planet from M. Le Verrier, he took advantage of a brilliantly clear evening which fortunately occurred on the same day, to direct his telescope to the spot communicated to him, when he soon noticed an object which was not inserted in the star-chart with which he was comparing the sky. He saw in about three hours that it had certainly moved, but on the succeeding evening he noticed that it had retrograded relatively to the neighbouring stars full four seconds of time. He then announced the discovery of the planet now known as Neptune, which has since been found to move in an orbit exterior to Uranus at a mean distance from the Sun of 2,760 millions of miles. Although the actual discovery of Neptune resulted from the calculations of M. Le Verrier, yet it was partly by accident that it was so. Mr. Adams had placed his approximate elements of the disturbing planet in the hands of the Astronomer Royal and Professor Challis many months previously; and the latter had even commenced a search for the planet on the 29th of July, 1846. As the star-chart used by Dr. Galle was unknown in England, Professor Challis was obliged to make his own maps. Unfortunately, these manuscript maps were only partially compared to show whether the plan of observation was effectual or not, for Professor Challis appears to have had no sanguine hopes at the time that "the indications of theory were accurate enough to give a chance of discovery in so short a time." After Dr. Galle's announcement, it was found that the Cambridge astronomer had actually noted the position of the planet in the map of the stars made on August 12th, and that when the previous partial examination had been made, the comparison was suspended at star No. 39, when the planet was No. 49 on the map. The diameter of Neptune is reckoned to be about 37,000 miles, but owing to the remoteness of the planet, this value is subject to some uncertainty. Its mass is about the 19,000th part of that of the Sun, and the density of its material is about twothirds of the density of the matter composing that body. On the surface of Neptune the Sun appears about the same magnitude as Venus when at its greatest brilliancy as viewed from the Earth; but the intensity of the Sun's light would be more than 10,000 times greater than that of Venus. As some compensation for the comparative absence of solar light, it is very probable that Neptune is furnished with several moons, in like manner with the other major planets. Our present telescopic means, however, have not sufficient space-penetrating power to enable us to detect many such minute objects at so great a distance. One satellite has undoubtedly been seen, first by Mr. Lassell, with his great reflecting telescope, in October, 1846, soon after the discovery of the planet. It was afterwards seen by M. Otto Struve, at Pulkowa, and by Professor Bond, of Cambridge, U.S., through the large equatorials of these two observatories. These instruments are the largest at present mounted, the object-glasses in each having a clear aperture of fifteen inches, the focal length being twenty-five feet. At one time a suspicion of a second satellite, and of an appearance similar to the rings of Saturn, was noticed by several astronomers, but it has not been confirmed. Mr. Lassell, who carried his superb twenty-foot reflector to Malta, to take advantage of the clear skies of southern Europe for deciding some of these delicate points of astronomy, has since declared his conviction that we have no authority to say that Neptune has more than one satellite, or that the supposed ring has any existence whatever. Neptune and Uranus have been several times observed as fixed stars. Uranus was seen at the Royal Observatory so far back as the time of Flamsteed, who recorded its position at least on six occasions, not knowing at the time that it was a planet. Dr. Bradley, at Greenwich, also saw it in 1748 and 1750. Neptune was unconsciously observed by Lalande in 1795, and by Dr. Lamont in 1845 and 1846, before its discovery was announced by Dr. Galle in the latter year. As viewed with the transit-circle at Greenwich, Neptune appears as large as an ordinary star of the seventh and a half magnitude. The intensity of its light is however much less than that of a star of corresponding size. COMETS. "Hast thou ne'er seen the Comet's flaming flight? Through depths of ether; coasts unnumbered worlds Of more than solar glory; doubles wide YOUNG. ROM the earliest ages, comets have been regarded with a superstitious awe which has never been attached to the planetary members of the solar system. Louis the First of France was so much alarmed by the appearance of one which became visible in the year 837, that he felt it necessary to satisfy his mind and to appease the wrath of Heaven by ordering the construction of a number of churches and monasteries, which his conscience told him he had hitherto neglected. It is related by Gibbon that "in the fifth year of the reign of Justinian, A.D. 531, and in the month of September, a comet was seen during twenty days in the western quarter of the heavens, and which shot its rays into the north. Eight years afterwards, while the Sun was in Capricornus, another comet appeared to follow in Sagittarius ; the size was gradually increasing; the head was in the east, the tail in the west, and it remained visible about forty days. The nations, who gazed with astonishment, expected wars and calamities from their baleful influence; and these expectations were abundantly fulfilled." In 1456, at a time when Christianity was struggling against Mohammedanism in Eastern Europe, the appearance of a great comet, which has been since identified as that of the celebrated periodical comet of Halley, excited the utmost consternation in Rome, where it was regarded as a portentous sign of approaching evils. Pope Calixtus III. thereupon ordered prayers to be said in the churches, and the bells to be rung daily at noon to call upon the inhabitants to supplicate the Divine protection during the impending calamity. The progress of education since the time of the Reformation in Europe has very nearly banished these superstitious fears from the minds of all classes. These occasional visitors to our skies are now so universally known to appear to us from natural causes, easily explained, that even the most brilliant of its class may be visible for months-the comet of 1811 for example—and yet the only excitement that may be noticed in the popular mind is that of extreme pleasure and delight at witnessing the progress of the wanderer across the heavens. Comets, next to meteors, are the most numerous of all the bodies which revolve around the Sun. Mr. Hind supposes that, since the Christian era, upwards of four thousand have approached the Sun within the orbit of Mars. Of this number, observations of 607 have been made in Europe and China up to the middle of the present century. The greater number of them have been visible to the naked eye, the telescope never having been much used in searching for them before the middle of the last century. Small telescopic comets are very common, and a year scarcely ever passes away without two or three, and sometimes more, being discovered. In short, although the general public are not aware of the fact, astronomers have usually one or more of these faint bodies on their list of observable objects. The head, or nucleus, of a large comet has mostly a central condensation of light surrounded by a nebulosity more or less luminous. When viewed through a lowpower telescope this nucleus appears tolerably well-defined, with a faint nebulous tail, generally of several degrees in length, streaming into space in a direction opposite to the Sun. When, however, a powerful telescope is employed with a higher magnifying power, the nuclei of no two comets are similar, and even that of the same comet will change from day to day, and sometimes from hour to hour. Changes have even been noticed while the nucleus has been under the eye of the observer. Comets perform their journey around the Sun in very eccentric orbits inclined to the ecliptic in all directions. The time occupied in their revolution is very different. The greater number have what is called parabolic orbits, which show that their motion during the time they have been under observation forms a part of a parabola. In these cases the period of revolution may extend over hundreds, or thousands, of years, and any prediction of their return is uncertain, but there is some probability of their reappearance at a distant period. Several have been observed to move in ellipses, and a few in hyperbolas. Comets which move in ellipses must return periodically at epochs more or less distant. Most of the brilliant comets which have been remarkable have parabolic orbits, that predicted by Halley to appear in 1759 being the only exception. The periodical comets are generally telescopic; so faint are they that it requires the greatest skill of the observer to detect them among the stars, and when found to fix their positions. They are, nevertheless, much more interesting to the astronomer than some of the grandest. That known by the name of Encke's comet is peculiarly so. This object was observed on several occasions between 1786 and 1818 with the belief that in each year a different comet had been observed. It was not till after the discussion by M. Encke of the observations made in 1818, that it was proved by that astronomer that this was a comet of short period, and that it revolved around the Sun in an eccentric ellipse in little more than three years. In 1822, 1825, |