The

upon the surface, an evident proof of the borrowed radiance of the orb. The first transit ever observed was predicted by Keppler, and witnessed by Gassendi, at Paris, on the 7th of November 1631. The second observed transit, on the 3d November 1651, was watched at Surat, in India, by Shakerley, a young Englishman, who, having found by calculation that it would be visible only in Asia, proceeded thither to witness the occurrence. third recorded transit, on the 3d of May 1661, was observed by Hevelius at Dantzic, and by Huyghens, Street, and Mercator at London, who are said to have made their observations at Long Acre, with a telescope of excellent workmanship. Halley enjoyed the sight of another, 7th November 1677, at St. Helena; and was the first who witnessed both the ingress and egress of the planet. Lalande, in his old age, remarked of the transit of 8th November 1802:-"The passage of Mercury over the sun's disk was observed this morning for the nineteenth time. Astronomers enjoyed in the completest manner the sight of this curious phenomenon. I was the more anxious to have a view of it, as I shall never see it more." The recent transits of Mercury have been chiefly interesting on account of the accuracy with which they have been predicted. Formerly, owing to difficulties of observation, the tables of the planet's motions were so imperfect, that there was no certainty respecting the exact time of the phenomenon. Gassendi was on the watch three days for the transit he witnessed. Hevelius and his assistants were kept waiting at their telescopes four days. Both Halley's and Lalande's calculations were wrong by three quarters of an hour with reference to the transit of 1786. At a later date, the errors amounted to minutes. At length, in 1844, Leverrier took the planet in hand, instituted a profound examination into the theory of its motions, and constructed tables which represent them with wonderful precision. Mr. Mitchel, of the newly-established observatory of Cincinnati, in the United States, remarks of the transit of 8th May 1845:-"Five minutes before the computed time of the contact (Leverrier's), I took my place at the instrument; the beautiful machinery that carries the telescope with the sun was set in motion; and the instrument directed to that part of the sun's disk at which it was anticipated the contact would take place. It seemed I watched until I as if time had folded his wings, so slowly did the moments crawl on. was told that but one minute remained; and within sixteen seconds of the time, I had the almost bewildering gratification of seeing the planet break the contact, and slowly move on till it buried itself, round, and deep, and sharp, in the sun."

VENUS. The nearest planet to the earth, and the second in point of distance from the Like Mercury, sun, Venus is the most beautiful of his satellites, and brilliant of the stars.

she never adorns the midnight sky, nor has she ever been seen rising in the east while the sun was setting in the west, or on the meridian at either sunrise or sunset. This shows her path to be comparatively near the throne of the great luminary, from whom she never departs more than 48°, rather more than half the space from the horizon to the zenith; and to be interior to that of the earth, while exterior to that of Mercury, whose greatest elongation, or distance from the sun, is little more than 28°. In addition to this, she has been observed to eclipse Mercury, a clear proof of her position in space being external to him, an instance of which occurred on the 17th of May 1737. Venus is alternately a morning and evening star, visible for about three hours after sunset, and as long before sunrise. As a morning star she was called Phosphorus and Lucifer by the ancients, and as an evening star Hesperus and Vesper. This bright herald of the sun's advance to the eastern horizon, and his faithful follower to the western, were once supposed to be distinct bodies. Pythagoras is said to have been the first who proclaimed their identity. Obvious as this conclusion now is, it required experience and reflection to arrive at it. The Greek Phosphorus, or the light-bringer, alludes to the office of the planet, when rising before the sun, she ushers in the day. The Romans adopted the expression: hence, the invocation in Martial, Phosphore, redde diem, "O Phosphorus,

restore the day." As an image of the anticipated dawn of full mental illumination, the sacred writers introduce the day-star; and it occurs as an emblem of mere human glory in the ode on the overthrow of the King of Babylon :

"How art thou fallen from heaven, O Lucifer, son of the morning!"

Homer compares the son of Hector to the star that gilds the morn, and the point of the spear of Achilles to the keen light of radiant Hesper. Men of all ages, climes, and ranks, from the shepherd boy to the grave philosopher, have turned with interest and delight to this planet; and whether examined as a telescopic object, or contemplated with the unassisted sight, it is an inexpressibly lovely orb, and one that will always excite admiration.

Venus is situated in the system at a mean distance of sixty-eight millions of miles from the sun, through which she would fall in thirty-nine days and a half, if surrendered solely to the attraction of his mass. Her periodical revolution is accomplished in 2243 days, involving a velocity of 80,000 miles an hour. The planet is rather smaller than our own globe, having a diameter of 7700 miles. She rotates upon her axis in about 23 hours. Venus exhibits alternately a fine thin crescent and a semicircle like the moon, but she can scarcely be seen quite full, because when the whole of her enlightened hemisphere is turned towards us, she is either behind the sun or so near him as to be hid by the splendour of his light. The diagram represents the various appearances of the planet as she moves in her orbit, in the order of the letters.

East elongation.

Superior conjunction.

S

West elongation.

Inferior conjunction.

We first behold Venus as a morning star for a short time before sunrise, soon after passing between the earth and the sun at her inferior conjunction, when she appears crescent-shaped as seen through a telescope. She continues gradually to gain upon the sun, rising earlier and earlier, until her greatest angular distance westward has been attained, when she exhibits a semicircle, and shines with great splendour. Then the planet begins to return towards the luminary, making the same daily progress as in separating from him, rising later and later, diminishing also in brilliance, owing to the overpowering solar glory, in which she is lost at the time of her superior conjunction, being then behind the sun in relation to us. A few days afterwards Venus becomes an evening star, and is seen a short time east of the sun after his setting. She soon seems to have fallen considerably behind him, and continues to depart farther and farther, setting later every night, until her greatest elongation eastward has been reached, when she again exhibits a bright semicircle. The course towards the sun is then resumed, until she comes between him and the earth, sets with him, and is invisible, owing to the whole of her enlightened side being turned away from us. In a few days the phenomena of the morning star are repeated. Venus is seen to keep on the same side of the sun for a period of about two hundred and ninety days together. This seems at first sight a singular anomaly, as it is a greater interval than that occupied by an entire circuit round him It is however at once accounted for by considering that the earth is proceeding at

the same time in the same direction, though at a slower pace. The planet accomplishes an angular motion of 1° 36′ per day, while the earth follows at the rate of 59', and is thus gained upon by only 37' daily. But the two planets will obviously appear to keep on the same side of the sun, until Venus has gained half her orbit in advance, or 180°. This it will require about two hundred and ninety days to effect, as the difference of their daily rate, 37′ × 290=10730′=180° nearly. It will be seen from the diagram, that at the time of the inferior conjunction, the unenlightened half of the orb is turned towards us. Did she present in that position her illuminated side, we should see the planet as a small brilliant moon, shining with twenty times her ordinary lustre, as she is then a hundred and sixty millions of miles nearer the earth than when at the opposite point. As it is, however, she is our best friend among the stars, the most radiant of the host, and has been observed to cast a clearly defined shadow.

This planet, like Mercury, transits the sun's disk, and then appears shorn of her beauty, under the form of a dull dark spot. The first instance of the phenomenon being seen by any human being occurred on the 24th of November 1639, under remarkable circumstances. It was observed by two youthful friends; Jeremiah Horrocks, at Hoole, near Liverpool, and William Crabtree, at Broughton, near Manchester. The great credit in the observation is due to the former of these individuals, who anticipated the event, and predicted the time. Upon Keppler completing his tables in 1627, he predicted that Venus would pass over the sun's disk on the 6th of December 1631. He died before the time arrived. The transit was looked for, especially by Gassendi, but it was not seen; and Keppler had distinctly announced that it might not be visible in Europe, as the planet would not be in contact with the sun till towards sunset. It is now well known to have taken place during the night between the 6th and 7th. But Keppler had stated, that after this period Venus would not be seen again upon the solar disk till the year 1761. This was an error. Horrocks suspected it on going over the tables, and comparing them with others. Having repeated his calculations, he found that Venus might be expected to enter upon the solar disk a little before sunset, November 24, 1639. It appears that the time of the transit was nigh at hand when the discovery was made, so that there was no possibility of giving general publicity to the expected event, in an age when communication was very slowly maintained.

Though not doubting his own calculations, Horrocks did not deem it prudent to trust implicitly to them. Accordingly, the day before the event was really expected, he watched the sun at intervals; but only ordinary appearances were noticed. The next day, a Sunday, he attentively observed the solar image from sunrise till the hour appointed for going to church—"higher duties," as he remarks, "which might not be neglected for these pastimes." Upon becoming at leisure, he resumed his observations; and at a quarter past three o'clock in the afternoon, his sagacity and industry were rewarded. "At this time,"

he states, "an opening in the clouds, which rendered the sun distinctly visible, seemed as if Divine Providence encouraged my aspirations, when, oh! most gratifying spectacle, the object of so many wishes, I perceived a new spot of unusual magnitude, and of a perfectly round form, that had just wholly entered upon the left limb of the sun, so that the margins of the sun and of the spot coincided with each other, forming the angle of contact." Owing to the near approach of sunset, his time of observation was limited to about half an hour. Meanwhile, in the neighbourhood of Manchester, the day opened inauspiciously, and continued gloomy to a late hour in the afternoon. Crabtree had abandoned all hope of being able to test the prediction of his friend, till just before going down below the horizon the sun broke through the clouds. Repairing to the room where he had made his preparations, he saw the round black spot with unspeakable delight; and, according to Horrocks, he was so struck with admiration at the spectacle, as to continue gazing upon

it, without attempting to take measurements, till it became impossible to do so, owing to the clouds returning. Both observers received the sun's image through a telescope in a dark room upon a piece of white paper.

The transits of Venus are of special interest from their rarity and great physical importance. They occur at intervals of eight years, a small section of individual life; but alternating with intervals of more than a century, during which whole generations pass away, thrones crumble, and dynasties change. They are of importance as the very best means of ascertaining the distance and volume of the sun, which supply data for determining the distances and magnitudes of the planets, and serve as a universal standard of astronomical measurement. The second recorded transit, in 1761, was well seen; but the discordance of the results obtained at different stations shook faith in the accuracy of the observations. For the third and last, in 1769, many of the European governments sent costly expeditions to various parts of the globe; and among others, Captain Cook was despatched to witness the phenomenon at Tahiti. The next transit will occur December 9, 1874; and though not visible in this country, it will be watched elsewhere with an intensity of scientific solicitude which no natural incident has ever yet excited, and with much better instruments than have before been used. Another will follow after an interval of eight years, December 6, 1882, which will be seen in England, but only partially, commencing near sunset. Venus will not again be seen upon the sun's disk through the whole of the next century, or till June the 8th, 2004.

But little is known of the physical constitution of Venus, owing to the intense splendour with which she shines. The existence of a considerable atmosphere is inferred from the appearance of a penumbral light round the planet during her transits, as well as from a faint radiance observed to stretch beyond her directly illuminated hemisphere. The line in the annexed uppermost figure marks the boundary of the direct influence of the sun's rays; and the upper and lower projections beyond it show the twilight, which is referred to atmospheric reflection. Variable and fleeting spots have also been repeatedly noticed, as in the second figure, which naturally leads to the supposition of an atmosphere charged with clouds and vapours, with water upon the surface, from which they are formed. The conclusion that she has mountains and valleys rests upon the fact that the edge of her enlightened part appears shaded, that her corners are sometimes obtuse, and present a luminous point apparently detached from the planet. Schroeter regarded this as the summit of a high mountain, illuminated by the sun after he had ceased to be visible to the rest of that hemisphere. If these conclusions may be depended upon, and they are warranted by strong evidence, Venus presents striking points of analogy to the constitution of the earth. An atmosphere reflecting light, the medium of sound, and a highway for "fire and hail, snow and vapour," a superficies exhibiting the diversities of land and water, hill and vale- these are some of her probable attributes, features expressing a family likeness to our globe, and indicating the action upon her surface of that mighty upheaving agency, which, in bygone ages, piled the Alps, and reared the ramparts of the Himalaya.

THE EARTH. We now come to the abode of Man- the cradle, the home, and the grave of our race for a period of six thousand years-the third planetary body in point of distance from the central sun, and the first in the system of which we have certain knowledge of its being dignified with the presence of an attendant orb. The introduction of our globe to a place among celestial objects involves an apparent contradiction; yet such is its real character in the constitution of the universe, and such is its obvious aspect as viewed away from its surface. To rustic ignorance it will seem a statement palpably absurd, that any affinity exists between the earth and the stars in the firmament. They are mere points of light in the sky, and have no perceptible dimensions, whereas our world appears of immeasurable extent, and exhibits no luminosity like theirs to the eye of sense. It seems, too, a perfectly inert mass. There is no movement discernible independent of that of the rivers flowing in their channels, the seas tossing in their bed, and the forests bending to the gale, while the celestial bodies appear in constant procession from place to place in the concave of the heavens. There is nothing, however, more susceptible of demonstration than that the obvious state of our globe is not its actual condition-that the apparently quiescent habitation of mankind is an unceasing traveller in space-that its opaque mass exhibits the same luminous aspect to the nearer planets which they present to us-and that in structure and economy the earth is in fraternal relationship to the celestial host, and may be denominated, with perfect propriety, a star. Physical science, in the three departments of astronomy, geography, and geology, deals with the mass of our globe. The former is chiefly concerned with its figure and magnitude, its atmosphere and motions.

To the eye the earth appears an immense plain stretching out in all directions to an indefinite extent. This was the current opinion of mankind respecting its form in early times. But a few simple facts prove the suggestion of the senses here to be erroneous. The limit of vision to the traveller upon an extensive level, or to the mariner at sea, is a well-defined circle of which the observer is the centre; and it may be geometrically

proved, that this circular horizon is a certain indication of the circular figure of the body to which it relates. In any direction in which a ship leaves shore, or approaches the coast, the vessel is observed as if gradually sinking in the ocean, or rising from it