As soon as astronomy had learnt to know its position, it began to suspect that this earth, with its sun, and moon, and planets, and comets-the whole solar system-is but a speck in the vast firmament of the heavens. The more men worked and thought, the stronger grew the conviction that Sirius, the little twinkling star, must be a sun, immensely brighter than our own. For they had tried in vain to find out his distance. In vain! The distance always came out infinite. The measuring line placed in the hand of man shrank into nothingness in respect to the whereabouts of the nearest of those little orbs, and astronomy retired abashed. Do you ask me what is the measuring line which man has in his hand to apply to the stars? I shall tell you that it is no small matter as men count smallness. It is two hundred millions of miles-a line long enough, you would think; yet this line actually shrank into nothingness so absolute, that, half a century ago, it seemed as hopeful to mount to the stars as to compass their distance with so puny a line. But the thing has been done at last, and triumphantly done. We know the distance of a few of the nearest stars now, pretty accurately, at any rate. And I propose to endeavour to convey an idea of how this knowledge has been attained.

Well, then, to begin at the beginning, the first, line to which all others are referred, the primary unit, is the yardmeasure, by which ladies' dresses are No. 27.-VOL. V.

measured-nothing more nor less. It does not concern us to enquire what that yard-measure is. Suffice it that the legislature provide means to prevent its fluctuation from year to year, or from century to century. Now, the yard can readily be multiplied to a considerable extent for example, into a chain of twenty-two yards-and with this chain a line of three or four miles can be measured on the earth's surface. The yard is thus expanded into miles. It is no easy matter, certainly, to measure a few miles on the surface of the earth; but it is possible, and has been done. An extension of this process would, of course, measure a very long line; but this is not necessary. Having once got over a few miles, the yard-measure, and the steel-chain, and all similar appliances are discarded, and the measured line itself is assumed as a new measuring-rod. True, it cannot be carried about from place to place. Mahomet cannot go to the mountain; so the mountain must be brought to Mahomet. This is done by making direction serve as the evidence of distance. If you measure off on the paper a line a foot long, and take a point somewhere over the centre of it, you will see how the angles of direction from the ends of the line depend on its distance from the line. So, conversely, if a church-steeple, or some other prominent object, be visible from both ends of the line measured on the earth's surface, its distance from either of them can be determined at once, by means of

N

angles, without approaching the object at all. You see then how we can get a good long line of sixty or seventy miles. Now, as the earth is a sphere or nearly so, if you travel due north a 360th part of the earth's circumference, you will find that the pole star has assumed a position one degree higher in the heavens. Accordingly, if you can measure distances and angles, the determination of the circumference of the earth is reduced to a matter of mere multiplication. The old Indians had got thus far; the old Greeks too. Two hundred and thirty years before the Christian era, Eratosthenes, the librarian of the Alexandrian library, observed the meridian height of the sun at Alexandria, at the time of the summer solstice, and then set to work to measure the distance up the Nile to Syene, where the granite quarries still show the marks of the chisel that cut out those wonderful obelisks from them. Here he found, or somebody found for him, a telescope ready to his hand-the earliest telescope on record. It was a reflecting telescope, like Herschel's, polished by nature's own machinery. The mirror was the surface of standing water, and the tube was one of those vertical shafts which, as in Joseph's well, have stood the wear of ages, and are wonderful even in the land of the pyramids and the sphinxes. Far, far down in the bowels of the earth, the brighter stars were visible by day. This telescope disclosed the fact, that Syene is just under the northern tropic. And so Eratosthenes, like his great benefactor Alexander, conquered the world. He did not weep because there were no more worlds to conquer; for were not the bright orbs, the allies of his first victory, like the Thebans, sure to become an easy prey to his chariot-wheels? But the work of Eratosthenes was done, and they gave him as a reward a mountain in the moon, which bears his name.

To be sure, the 250,000 stadia which Eratosthenes estimated as the circumference of the earth, was a rough enough approximation as compared with the precision of modern times. But it was a great work for one man. Since then,

the nations of Europe have set themselves to the task. One instance deserves mention.

In 1791-2, the National Convention of France conceived the magnificent idea of establishing a new standard for everything-morals, money, and measure. "Let the heavens," they said, "furnish 66 new units of time, and the earth new

"units of space. Let the week, and "the month, and the year yield up their "ancient prerogatives. Let the former "history of the world be forgotten, and "let all history date from this time. "Let the month be divided into thirty

66

days, and let the sabbath occur every "tenth day. Let the day be divided "into ten hours, and let new dials be "constructed to show them. Let a "girdle be drawn round the earth, which "shall connect Paris with the poles: "let this girdle be the standard of 66 measure, and let men be sent out to "ascertain its amount." A magnificent order, truly! Yet it does seem easy enough to count by thirties and by tens -to make the month thirty days, and the week ten; but to measure the circumference of the earth, this is a work, a labour! It so happened, however, that the thirty days, and the new sundials, and the unscriptural sabbaths failed to struggle into existence-a higher power protected France from herself; whilst the measure of the meridians-a work beset with appalling difficulties was accomplished; and the mètre, the ten-millionth part of the measured quadrant of the earth's circumference, is the national standard throughout France to this day.

M. Arago, in his autobiography, gives an amusing, but perhaps an exaggerated, sketch of his own share in these labours. He tells us that he commenced by pacing to and fro, for the space of six months, on the narrow platform of a rock which overlooks the Mediterranean, to watch for the signal-light from the island of Iviza. From this airy spot he was transferred to the closer atmosphere of the castle of Belver, wounded, and a prisoner. Here he had the satisfaction of reading in the Spanish papers a detailed account of his own execution. Judging that the announcement was but the prelude to the event, he looked about for the means of escape. From the window of his prison he finds he can leap into the sea, and he resolves on doing so; conceiving, as he says, "that it is as well to be drowned as to be hanged." But he is not drowned. He reaches a ship, and is conveyed to the coast of Africa, where he finds the Moors almost as uncivilized as the Spaniards. So he is not sorry when he is allowed to return to his work. Once more in Spain, he is not long in discovering that brigandage is one of the institutions of the country. His temporary station, on the top of a mountain near Culléra, is visited, one stormy night, by the chief bandit of the district. The astronomer makes him his friend, and the work proceeds merrily under his protection.

Enough. We have measured the earth, but we are a great way from the stars still. Our yard measure has brought us thousands of miles on our journey; but the stars are millions of millions of miles away, and how are we to get at them? We shall see. Remember, then, that, when we had a base line of a few miles, we could determine the distance of an object seen from either end, by means of angles alone. In the same way, we get at the distance of the sun, or of a planet, by the longer base-line of the earth itself. We get at it roughly, it must be confessed. Copernicus, Tycho, even Kepler himself, had no idea that the sun is so far from us as he really is. Had the sun been fixed im

movably in the heavens, it might have been easy, or, at least, it might have been deemed easy, to compare his distance with the size of the earth. But the sun wanders among the stars and rolls round the earth, and thus seems to defy the efforts of the measurer. It was the good fortune of James Gregory to point out a method by which his distance may be determined, spite of his unsteadiness. The orbits of the two planets, Mercury and Venus, lie between the sun and the earth, so that those planets occasionally cross the face of the sun-Mercury frequently, Venus more rarely. It occurred to Gregory that observers at different parts of the earth's surface would witness a transit across different parts of the sun-one seeing it cross the centre, another observing it graze the edge. And, as the time it took in crossing might be readily ascertained in either case, the places at which it crossed would be thereby determined. And thus, knowing the positions of the two places of observation, and the corresponding positions of the projection of the planet on the sun's disk, the determination of the distance of the sun would, by a little help from theory, be reduced to a mere matter of triangles. Perhaps Gregory hardly appreciated the full value of the suggestion he was making. At any rate, nothing followed the publication of his hint for a great number of years. At length, about the beginning of the last century, it assumed, in the mind of Halley, the definite and practicable form which renders it now the corner-stone of astronomy. Halley perceived that the planet Venus was greatly to be preferred to Mercury for the determination of the sun's distance from the earth. His lucid statements and earnest exhortations aroused the whole astronomical world, and a transit of Venus was anxiously awaited. Halley himself, indeed, when he directed attention to the importance of the method, had no hope of living to see it tested. He stood like Moses on the top of Pisgah, and looked on the Promised Land; but to cross the Jordan was not his earthly lot. He had been laid with

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 remaining at Pondicherry until 1769

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 that! You cannot think of such dis

tances as these-the mind is unable to grasp them. Dobrizhoffer, the Jesuit missionary, tells us that the Abipones of Paraguay, amongst 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 Professor 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 distances of the stars thirty or forty years

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. The 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 system. This fact, and not their size,