his fathers many a year before the occurrence of the transit from which he had prepared men to expect so much. At length, in 1761, the looked-for time arrived. Now transits, which are of very rare occurrence, when they do happen, occur in pairs, at an interval of only eight years. Thus, when, after anxious waiting, astronomers beheld the transit of 1761, they knew that in eight years they should witness another. It was probably this circumstance of a second transit to fall back upon that rendered the observations of 1761 so little worth. That date being past, and the occasion lost, the succeeding transit of 1769 was all that the world had to rely on for another century. Had this opportunity been again lost, what a different position would our astronomy and our navigation have been in from that which they now occupy! Happily, all Europe was astir. Men were sent out north and south, east and west, to make the whole length and breadth of the globe available base - lines. England fitted out an expedition to the South Seas, and placed it under the command of Captain Cook. Who has not read Cook's first voyage? Most of us have devoured it, every part but the account of the observation of the transit, the real object of the expedition. Possibly it would have been otherwise had the astronomer Green returned to tell his own tale. But it was not so to be. His body was consigned to the deep during the homeward voyage. But his

observation was made under favourable circumstances, and is invaluable. In this respect, Green was happier than some of his fellow - labourers. The Abbé Chappe erected his observatory in California, and died ere his work was well complete. M. Le Gentil had been sent out to Pondicherry to observe the previous transit of 1761; but the winds and the waves detained him on shipboard until after the event had taken

place. But Le Gentil was a man of spirit, not easily discouraged. Accordingly, he resolved to lessen the chance of a second disappointment, by

for the second transit. But, alas ! alas ! after eight years of weary waiting, a little cloud effectually hid the phenomenon from his sight, and Le Gentil had to return to France empty as he left it. Poor Le Gentil! for him there is no cross of honour in life, no national monument at death. He is like the poor subaltern who leads the forlorn hope, and perishes in an unsuccessful attack. Let us drop a tear to his memory and that of Green ere we proclaim that the stronghold has fallen!

The solar system is now measured. The distance of the sun is now ascertained with positive certainty. Seven different base-lines, a host of independent observations, all concur in giving the distance of the sun from the earth (in round numbers) as ninety-five millions of miles. It is a grand era in astronomy. What would Copernicus, what would Tycho have said? They, worthy men, great astronomers as they were, never dreamt that the sun is a tenth part as far away. Even Halley, when he proposed this most successful problem, laboured under the delusion that he was some thirty millions of miles nearer the sun than he actually was.

Well, we have extended our yardmeasure to a pretty good length now. As the earth goes round the sun every year in an orbit nearly circular, the position we shall occupy six months hence will be just a hundred and ninety millions of miles from where we now are. And we can observe a star from both ends of this line, just as we observed a steeple previously from the two ends of a field. Our measuring tape for the stars is a hundred and ninety millions of miles. Yet, great as this distance is, so inconceivably far away are the stars, that all the refinements of modern science were unable, half a century ago, to deduce anything about them but this negative conclusion-that the nearest of them is at least a hundred thousand times as far from us as spring is from autumn, or summer from winter-a hundred thousand times a hundred and ninety millions of miles; no star nearer than

tances as these-the mind is unable to grasp them. Dobrizhoffer, the Jesuit missionary, tells us that the Abipones of Paraguay, ainongst whom he laboured, have no better mode of expressing numbers. above a score or so, than by taking up a handful of sand or grass and exhibiting it. They had to pass through a deal of schooling to learn. to count up to a thousand. The Pro

fessor at Angers, wishing to exhibit to his class the relative magnitudes of the sun and the earth, poured sixteen pecks of wheat on his lecture table. "This," said he, "represents the sun, and one of the grains represents the earth." If we try a similar method, we shall not succeed so well. Let us, however, try. You have some faint idea of three thousand miles, from having painfully measured it on the Atlantic, it may be. The thirtieth of an inch, on the other hand, you can estimate well enough. It is the dot you place over the letter i, as you write. Well, suppose this dot to represent the distance between Liverpool and New York; then will the actual distance three thousand miles-represent the interval, nearer than which there is no fixed star. Three thousand miles of dots, when each separate dot stands for three thousand miles! you may help your mind, or cheat yourself into the belief that you do so, by some such process as the following. Light travels with such a velocity, that it would fly round the earth, at the equator, eight times in a second. Yet there is no star so near us, but that its light occupies more than three years on its journey to the earth. The whole starry firmament, seemingly so bright, may, for ought we know, have been quenched in everlasting darkness, three years ago. Were such a catastrophe conceivable, the lamps of heaven would go out, one by one, to mortal eyes, year after year, and century after century, until, some two thousand years hence, the faint light of stars of the sixth and seventh magnitude would alone hold on its journey.

Or

All that was known about the dis

The

ago, was this negative fact. No star nearer than the parallactic unit, as it is called, of twenty millions of millions of miles! Whether any were so near, or anything approaching the distance, nobody could say. At length the question of distance was resolved. And here occurs one of those singular duplications-twins in the births of thought -with which the history of science abounds. The first determination of the distance of a star from the earth was worked out simultaneously by two men, under circumstances which precluded the possibility of mutual assistance; and the results were presented to the world within a few days of each other. memoir of Bessel, which announced a sensible parallax for 61 Cygni, appeared on the 13th of December, 1838. That of Professor Henderson, in which the parallax of a Centauri was established, was read to the Astronomical Society on the 6th of January, 1839, and had of course been in the hands of the Society some days previously. There was no desire on the part of either astronomer to contest the claims of the other. Many years subsequently it was my good fortune to unite with Professor Henderson in entertaining his illustrious friend, Bessel; and it was a gratifying sight to witness the warmth of affection with which these two good men welcomed each other as fellow-workers in the same field. They have both gone to their rest-Henderson too early for science; Bessel at an advanced age, and full of honours.

The stars which Henderson and Bessel selected were in one respect very unlike. That of Henderson is a bright star in the southern hemisphere; that of Bessel is a faint inconspicuous star in the northern. But the stars have one thing in common-both have large proper motions. They are not fixed stars, in the strict sense of the word; they move on by a few seconds annually. And this circumstance of a proper motion was an argument in the minds of the astronomers, that those stars are in close proximity to our

was the ground on which they were selected. Professor Henderson commenced his calculations with a different object, and only diverted them into the channel of distance when he ascertained the amount of proper motion which the star has. His observations were not undertaken with a view to this question; they were ordinary meridian observations. And it is not to be wondered at that astronomers were very cautious in admitting results so obtained, when it is considered that observations of this kind are beset with such numerous sources of error, in refraction, aberration, and the like. The method adopted by Bessel, on the other hand, obviates those sources of error. It has some analogy to the method of obtaining the distance of the sun by means of a transit of Venus, inasmuch as the observations are not those of the absolute position of one body, but of the relative positions of two.

The basis on which the operations are conducted is this:-Certain stars are so nearly in the same direction in the heavens as not to be easily separated. Some of these are in reality doubletwin stars revolving about each other— at any rate, physically connected. Others have no such connexion; and it is argued that, in certain cases, the smaller of the two is likely to be at an enormous distance behind the other. When such is actually the case, there will be a change of the relative positions of the two as viewed from different parts of the earth's orbit, and the amount of that change will depend on the proximity of the nearer star to our system, in precisely the same way as a tree will shift its place more or less rapidly, with respect to a distant hill, as the spectator is carried along in his journey. It is on stars so circumstanced that observations with the view of detecting a parallax were instituted by Bessel. No absolute measures of position of either star are required; simply the relative distances and directions of the one with respect to the other. Thus all sources of error due to refraction, aberration, and many other causes, which equally effect both stars,

The conclusion may be stated in a single sentence. The star selected by Henderson is only a little beyond the parallactic unit (twenty millions of millions of miles); that selected by Bessel is about three times as far away. Other stars have been reached, but these two are the nearest known. With a trembling and uncertain hand, astronomers have stretched out their line to one or two stars ten times as far away as the farthest of these. But the great host of heaven lie incalculably farther back. Shall we ever reach them? Judging from present appearances, we are compelled to answer in the negative. The stars, as we gaze into the sky, seem to defy us. For what do we see there? Close around us we see bright lamps pretty equally distributed over the vault of heaven. They twinkle and dance before us, as though conscious of the close proximity of our gaze. But let us look again. Clasping the whole vault of heaven, we see a belt of faint light, some twelve degrees in breadth. This is the milky way, the galactic circle. To the ancients, it was part of the milk which washed the purple stains from the lily; to the moderns, it is the universe itself-the stupendous whole, of which the brighter stars are but the portions which lie nearest to this little spot of earth. You may understand this if you bear in mind that the spherical appearance of the heavens is a necessary consequence of vast and unknown distance. There is no reality in this appearance. The arrangement of the stars is somewhat like an extended sheet of cardboard, of small thickness. Or, rather, you should imagine a vast plain planted with orange trees, all loaded with yellow fruit. These oranges in countless myriads are the stars. We are situated near the centre of this grove. Our sun is a small orange; the earth and the planets are tiny buds grouped around it. The neighbouring branches are thinly supplied with fruit, and few fruit-stalks bear more than a single orange. But the grove is of boundless extent. Look