source of life, and light, and heat, demands the attention of the student of the heavens. That some inscrutable tie binds it to the earth, or the earth to it, was early recognised in the fact, that whether the sun was moving or at rest, the relative distance of it and the earth never changed by any great amount; and whatever changes did occur, were all obliterated in a short period, and the distance by which these bodies are separated was restored to its primitive value, to recommence its cycle of changes in the same precise order. Here, then, was a grand problem to determine the relations existing between the sun and earth; to endue with motion that one of these bodies which did move; and to fix the limits within which the observed changes occurred, both in time and distance.

While the connection between the sun and earth was certain, a mutual dependence between the earth and the other great source of light, the moon, was equally manifest. The invari ability in the apparent diameter of the moon demonstrates the fact, that whether the earth were moving or stationary, the moon never parts company with our planet. In all her wanderings among the fixed stars, in her elongations from the sun, in her wondrous phases and perpetual changes, some invisible hand held her at the same absolute distance from the earth. But to decide whether this power resided in the earth or the moon, or in both; to explain these wondrous changes from the silver crescent of the western sky to the full orb which rose with the setting sun, pouring a flood of light over all the earth; to develop the mysterious connection between the disappearance of the moon and those terrific phenomena, the going out of the sun in dim eclipse; these furnished themes for investigation, requiring long centuries of patient watching, and of never-ending toil.

Passing out from the sun and moon to the more distant stars, among the brightest of those which gemmed the nocturnal heavens, a few were found differing from all the rest in the fact that they wandered from point to point, and, at the end of intervals widely differing among themselves, swept round the entire heavens, and returned to their starting point, to recommence their ceaseless journeys. These were named planets, wanderers, in contradistinction to the host of stars which were fixed in position, unchanged from century to century.

Hence arose

a new and profound series of investigations :

Where were these wandering stars urging their flight? Were their motions real or apparent? Were their distances equal or unequal? Did any tie bind them to the earth, or to the sun, or to each other? Were their distances from the earth constant or variable? Were their motions irregular, or guided by law? Did they accomplish their revolutions among the fixed stars in regular curves, or in lawless wanderings? Among all the moving bodies, sun, moon, and planets, could any principle of association be traced which might bind them together, and form them into a common system?

To resolve these profound questions, a critical watch is kept on all the moving bodies. Their pathway is among the stars, and to these ever-during points of light their positions are constantly referred. If beyond the limits of the moving bodies a dark veil had been drawn so as to have excluded the light of the stars, at the first glance it might seem that by such a change simplicity would have been introduced, and the perplexity arising from the motion of the planets among the profusely scattered stars would have been removed. But let us not judge too hastily. Blot out the stars, and give to the sun, moon, and planets, a blank heavens in which to move, and the possibility of unravelling their mysterious motions, mutual relations, and common laws is gone for ever.

This will become manifest when we reflect that on such a change, not a fixed point in all the heavens would remain, to which we could refer a moving planet. They must then be referred to each other, and the motion due to the one, would become inextricably involved in that due to the other, and neither could be determined with any precision. Like the ocean islands which guided the early mariners, so God has given to us the stars of heaven as the fixed points to which we can ever refer, in all parts of their revolution, the places of the wandering planets, and the swiftly revolving moon.

As the necessity for accuracy in watching the movements of the planets became more apparent, the attention was directed to the acquisition of the means by which this might be accomplished. Hence we find in the earliest ages the astronomer grouping the fixed stars into constellations, breaking up the great sphere of the heavens into fragments, the more easily to study its parts in detail. Not only are the stars of each constellation numbered, their brilliancy noted, but their relative places in the constellation

and to each other are fixed with all the precision which the rude means then in use permitted. Names are fixed to these different groupings; when, or where, or by whom, we know not. Neither history nor tradition leads us back to this first breaking up of the heavens, but the names then bestowed on the fragmentary parts, the richer constellations, have survived the fall of empires, and are fixed for ever in the heavens.

Chaldean Shepherds naming the Constellations.

Possessing now a thorough knowledge of the objects among which the planets were moving, and the means of measuring with approximate accuracy, their distance from the stars along their path, it became possible to trace a planet in its career, and to note the changes of its velocity.

New and wonderful discoveries were thus made. It was found that all the planets moved with an irregular velocity. Sometimes swiftly advancing among the fixed stars, then slowly relaxing their speed, they actually stopped, turned backward in their career, stopped again, and then, at first slowly, but afterwards more rapidly, resumed their onward motion. These strange and anomalous motions, differing from anything remarked in the sun and moon, furnished new themes for discussion, new problems for solution. While the phenomena above alluded to became known, the same chain of observations revealed the remarkable fact, that the periods of revolution of the planets, though differing for each one of the group, were identical for any one individual; and moreover, that a simple curve marked out the pathway of sun, moon, and planets, among the fixed stars, and that all these wandering bodies were confined to a narrow zone or belt in the heavens.

Centuries had now rolled away, nay, even thousands of years had slowly glided by, since the mind had first given itself to the examination of the heavens; and while discovery after discovery had rewarded the zeal of the observer in every age, yet the grand object of research, the distinction between actual and apparent motion, had thus far eluded the utmost efforts of human genius. But a brighter day was dawning. Each successive effort tore away some petty obstruction which impeded the march of mind upward towards the lofty region of truth. Facts grew and multiplied. Phenomena, striking and diversified, were collated and compared. The mind in imagination took leave of the earth as the centre of all these complex movements, inexplicable on its surface, and naturally urged its flight towards the sun. There it paused and rested, and from this fixed point looked out upon the circling orbs, and lo! the complexity of their movements melted away. The centre was found, the mystery solved, the ponderous earth rescued from its false position, rolled in its place among the planets, one of the great family that swept in beauty and harmony about their common parent the sun.

The mind now stood upon the first platform of the rocky pyramid which it had been slowly rearing, and with which it had been slowly rising, through long centuries of ceaseless toil. One grand point had been gained. Darkness had given way to light, but the great problem of the universe was yet to be

resolved. All this long and arduous struggle had only revealed what the problem was. Appearances were now separated from realities, and with a fresh and invigorated courage the human mind now gave its energies to the accomplishment of definite objects, no longer working uncertainly in the dark, but with the clear light of truth to guide and conduct the investigation.

Possessed of these extraordinary advantages, the advance now became rapid and brilliant, as it had previously been slow and discouraging. That the planets, reckoning the earth as one, constituted a mighty family of worlds, was now manifest; whether linked singly to the sun, or mutually influencing each other, was the grand question. This great problem rested upon the resolution of a multitude of subordinate ones. The actual curve constituting the planetary orbits, the magnitude of these orbits, their actual position in space, the values and directions of their principal lines, the laws of their motion, all these and many more questions of equal importance and intricacy presented themselves in the outset of the examination now fairly commenced. Human skill was exhausted in the contrivance and construction of mechanical aids by which the movements of the planets might be watched with the greater accuracy. Partial success crowned these extraordinary efforts, but there yet remained delicate investigations which with the utmost skill in observing escaped the farthest reach of man's eagle gaze, and seemed to bid defiance to all his powers.

To conquer these difficulties, one of two things must be accomplished; either man must sweep out from earth towards the distant planets, to gain a nearer and more accurate view, or else bring them down from their lofty spheres to subject themselves to his scrutinising gaze. How hopeless the accomplishment of either of these impossible alternatives! But who shall prescribe the limits of human genius? In studying the phenomena of the passage of light through transparent crystallised bodies, a principle was discovered which let in a gleam of hope on the disheartened mind. It seized this principle, converted it to its use, and armed itself with an instrument more wonderful than any that fancy in its wildest dreams ever pictured to the imagination. With the potent aid of this magic instrument, the astronomer was no longer bound hopelessly to his native earth; without indeed quitting in person its surface, his eye, gifted with superhuman power, ranged the illimitable fields of space. He visited the moon,