a dark line, called the "black-drop," has been seen to shoot out from it, or the edge of the planet nearest the solar edge becomes elongated or pear-shaped. At the egress it is observed before the apparent internal contact. This phenomenon was first noticed in the transit of Venus of 1761, and again in 1769. In transits of Mercury it has also been frequently observed, but in the latter the effect is less in proportion to the difference in the magnitudes of Venus and Mercury. During the last transit of Mercury on the morning of November 5th, 1868, the formation of the black-drop preceding the apparent internal contact of the edges of the Sun and planet at the egress, was very clearly observed at the Royal Observatory, Greenwich. In the telescope of the great equatorial, the phenomenon was first seen as a thin dark filament stretching across the luminous line still existing between the limbs of the two bodies. In the altazimuth telescope, the existence of the black-drop was indicated by the planet assuming an elongated or pear-shape, tapering almost to a point at the Sun's edge. In smaller telescopes, the same phenomenon, under different phases, was exhibited. Its cause may be explained by supposing that the great irradiation, or spreading out, of the intense light of the Sun, cuts off or hides from view, during the transit, a portion of the planetary disk; that part which is hidden becoming suddenly visible as a dark line as soon as the planet reaches the true edge of the Sun unaffected by irradiation, which occurs when to the eye a luminous thread seems still to separate the two edges. The remaining transits of Mercury during this century will take place on May 6th, 1878; November 7th, 1881; May 9th, 1891; and November 10th, 1894. VENUS. When the planet Venus is near its extreme eastern or western elongation, no fixed star can bear the least comparison with it for splendour. When an evening star in the western sky, universal attention is always directed to it on account of the extra lustre it gives to that portion of the heavens. In the morning hours, however, Venus appears a still more magnificent object, owing principally to the clearer state of the atmosphere before sunrise. At the times of greatest brilliancy the light of Venus is very intense. A sensible shadow is often thrown upon a piece of white paper by the interposition of the hand between it and Venus when the planet is in this position of its orbit. It can also be plainly perceived by the naked eye at such times in full sunlight, sometimes within an hour of noon. At one of these epochs, in 1868, a correspondent of the Times fancied that he had discovered a balloon-shaped comet at noon-day by means of a small hand-telescope. The stranger, however, turned out to be the planet Venus, which happened to be favourably situated for daylight observation in the spring of that year. Venus was named by the ancients Hesperus and Lucifer, or the evening and morning star, names by which it is frequently identified, especially by our poets. Jupiter shares with Venus the popular appellation of the morning and evening star, and when in an elevated position in winter its lustre rivals that of Venus. Milton speaks of the latter as the "Fairest of stars, last in the train of night; If better thou belong not to the dawn. Sure pledge of day, that crown'st the smiling morn Venus is nearly of the same magnitude as the Earth, its diameter being about 7,500 miles. The sidereal period, or time of revolution round the Sun, is 224d 17h, and the time of its rotation on its own axis is about 23h 21m. Its orbit, which deviates but little from a circle, is nearly midway between those of the Earth and Mercury, the mean distance from the Sun being 66,431,000 miles. When nearest the Earth at inferior conjunction Venus is 25 millions of miles from us. As a telescopic object this planet is far too brilliant to allow the markings on its surface to be distinctly seen; but some astronomers have perceived ill-defined dusky spots, from which the assumed time of rotation has been determined, although much doubt has been expressed on the permanency of the spots used by Cassini in 1667 for that purpose. Sir W. Herschel occasionally saw spots on the disk of Venus, but he could not bring his mind to believe that they were anything but optical delusions. "For," he remarks, "the spots assumed often the appearance of optical deceptions, such as might arise from prismatic affections, and I was always very unwilling to lay any stress upon the motion of spots that either were extremely faint and changeable, or whose situation could not be precisely ascertained." Many observers have, however, since scrutinised the planet very closely, including Schröter, Beer, Mädler, and De Vico, all of whom confirm in some measure the early observations of Cassini. Viewed through a telescope, Venus exhibits phases of remarkable distinctness. Galileo, in 1610, was the first person who saw this phenomenon. It appears from a letter still in existence, that Father Castelli, a celebrated philosopher at Florence, asked Galileo if Venus and Mars ought not to present phases similar to the Moon. The idea does not appear to have crossed the inquiring mind of Galileo, or, if it had, he was cautious in propagating new astronomical facts, knowing full well the consequences, for Father Castelli received the following laconic reply:-"I am now occupied in so many researches, and the state of my health is so unsatisfactory, that I find myself far better lying in bed than exposing myself to the damp and chill of out of doors." Within six weeks, however, Galileo found time and inclination to explore the heavens with a telescope newly constructed, and was rewarded by the view of the crescent of Venus, which he announced to Castelli on December 30, 1610. The telescopic appearance of Venus may be likened to a miniature Moon. In short, their phases are produced by causes perfectly similar, depending on their relative positions in their orbits, with respect to the Sun and Earth. Venus is therefore sometimes round, then gibbous, then like a half-moon, and finally a crescent, which directly before inferior conjunction becomes so narrow as to appear like a curved illuminated hair. The crescent form is exhibited from the time of inferior conjunction to that of greatest elongation, when the planet becomes half-illuminated. Between the greatest elongation and superior conjunction, it assumes a gibbous form, becoming quite round when in a direct line beyond the Sun. The apparent magnitude of Venus varies to a considerable amount in the course of its revolution in its orbit, as its distance from the Earth increases or diminishes. Although Venus is now and then comparatively so near us, we know but little of its actual surface, principally owing to its intense brilliancy, which dazzles the eye of the observer. With regard to the two planets, Mercury and Venus, it is known that they are globes formed similarly to our Earth, and equally illuminated and warmed by the Sun. It is believed, also, from special observations of the physical appearance of the surface of Venus, that clouds prevail; if so, there must be water, and probably an atmosphere. This hypothesis would seem to be partially borne out by a phenomenon observed during the transit of Venus across the Sun's disk in 1761. While projected on the Sun, the planet appeared surrounded by a faint nebulous ring, and at the moment when Venus left the Sun, a luminous ring was observed on the Sun's edge. These two phenomena could be easily explained, if we suppose the globe of Venus to be surrounded by a very dense atmosphere. Farther than this, the most powerful instrument of the astronomer is unable to add to the little knowledge we possess of the actual formation of these intra-terrestrial planets. Of their peculiar motions in the heavens with respect to the fixed stars, and their effects on each other by their mutual attractions, the results obtained from modern astronomical observations leave but little more for us to learn. Neither Mercury nor Venus has any satellite visible through the most powerful telescope yet constructed. If Venus had an attendant which could bear any comparison with the Moon, it would most probably have been detected long before this, by some of our modern astronomers. Some of the most vigilant observers have, under every favourable circumstance of atmosphere, watched the planet for this purpose, but without success. Little faith can therefore be placed in the observations of the elder Cassini, Short, Montaigne, Horrebow, Montbarron, and others, who have recorded their opinions that on several occasions they obtained a view of such a satellite. Professor Lambert even collected all the reputed observations, from which he succeeded in computing a tolerably consistent orbit, the period of revolution being rather more than eleven days, and the distance from Venus nearly the same as that of the Moon from the Earth. M. Guillemin remarks that "Venus is very similar to the Earth in many points, both in its dimensions and physical constitution. If we accept the great number of observations made in the seventeenth and eighteenth centuries, it would have still another point of resemblance. As the Moon accompanies the Earth, Venus would be also provided with a satellite. But this singular body cannot now be seen, and high scientific authorities have declared that the observers had been deceived by an optical illusion. It must be acknowledged that the doubt which still exists on this account is at least very curious, and shows that in the domain of planetary astronomy there still remains a disputed point to be cleared up." The celebrated astronomer, Christianus Huygens, made some remarks in the second half of the seventeenth century on the planet Venus, which exhibit very clearly that our knowledge of the physical constitution of its surface has very little increased since his day. "The Sun," he observes, "appears to the inhabitants of Venus by half larger in his diameter, and above twice in his circumference, than to us, and by consequence affords them but twice as much light and heat, so that they are nearer our temperature than Mercury. Their year is completed in seven and a half of our months. In the night, our Earth, when 'tis on the other side of the Sun from Venus, must needs seem much larger and lighter to Venus than she doth ever to us; and then they may easily see, if they have not very weak eyes, our constant attendant the Moon. I have often wondered that when I have viewed Venus at her nearest to the Earth, when she resembled an half-moon, just beginning to have something like horns, through a telescope of forty-five or sixty feet long, she always appeared to me all over equally lucid, that I can't say I observed so much as one spot in her, though in Jupiter and Mars, which seem much less to us, they are very plainly perceived. For if Venus had any such thing as sea and land, the former must necessarily show much more obscure than the other, as any one may satisfy himself, that from a very high mountain will but look down upon our Earth. I thought that perhaps the too brisk light of Venus might be the occasion of this equal appearance; but when I used an eye-glass that was smoked for the purpose, it was still the same thing." To this testimony of the old astronomer, little can be added even now. The change of the position of Venus with respect to the stars is very rapid when approaching to or receding from inferior conjunction. It may interest the reader to see the amount of this change at sight, which may be done by an inspection of the diagram. This diagram is intended to show the apparent motion of Venus in the heavens from March 1st to September 15th, 1868, and includes the period of greatest elongation, when the apparent motion is the minimum. In the interval of time included between the extreme dates, the planet has passed over more than a quarter of the heavens. The figures above the diagram represent the right ascension, or the distance in time from the vernal equinox, or, as it is technically called, the first point of Aries. From this point all angular distances in this direction, or right ascensions, are measured along the celestial equator. The figures at the side represent the declination, or the angular distance measured perpendicularly from the celestial equator. Right ascensions and declinations serve the same purpose for distinguishing the positions of celestial objects as longitudes and latitudes define the positions of places on a terrestrial globe or map. THE EARTH. The Earth, treated as a celestial globe, is a planet travelling in space in the same manner as the other members of the solar system. To the inhabitants, if any, of Venus or Mars it would appear a brilliant object in their skies, rivalling in lustre any other planet. From the Moon, it would appear to great advantage, casting so much light as thirteen Moons united. Its form is not a perfect sphere, the diameter from the north to the south pole being smaller than the diameter at the equator by about the three-hundredth part. A proof of the spherical form of the Earth is shown by the gradual appearance or disappearance of ships at sea. If we were to place ourselves on an elevated position near the sea, the first signs of an approaching vessel would be the tops of the masts, then the upper sails would gradually become visible, to be succeeded by the lower sails, and finally the hull would come into view. Again, if we are journeying on the ocean towards the southern hemisphere, we might daily watch the gradual declension of Polaris towards the north horizon, or if proceeding northwards, it would be observed as gradually increasing its altitude from day to day. Now this star, as we have before observed, is so near the true pole of the celestial sphere, that its position with respect to the zenith remains nearly the same throughout the year, if our station on the Earth be the same. If the Earth were not a globe, could it be for a moment supposed that the apparent declension or elevation was produced by a real increase or diminution in the absolute distance between us and the star? Such an argument would be positively absurd, because we know that the position of the fixed stars never varies sensibly, even when viewed from opposite sides of the Earth's orbit, a distance amounting to more than 183,000,000 miles. When, therefore, we observe Polaris, or any other star on the |