Part 29 (1/2)
”The past year has given to us the new [minor] planet Astraea; it has done more--it has given us the probable prospect of another.
We see it as Columbus saw America from the sh.o.r.es of Spain. Its movements have been felt trembling along the far-reaching line of our a.n.a.lysis with a certainty hardly inferior to ocular demonstration.”
It was about time to begin to look for it. So the Astronomer-Royal thought on reading Leverrier's paper. But as the national telescope at Greenwich was otherwise occupied, he wrote to Professor Challis, at Cambridge, to know if he would permit a search to be made for it with the Northumberland Equatoreal, the large telescope of Cambridge University, presented to it by one of the Dukes of Northumberland.
Professor Challis said he would conduct the search himself; and shortly commenced a leisurely and dignified series of sweeps round about the place a.s.signed by theory, cataloguing all the stars which he observed, intending afterwards to sort out his observations, compare one with another, and find out whether any one star had changed its position; because if it had it must be the planet. He thus, without giving an excessive time to the business, acc.u.mulated a host of observations, which he intended afterwards to reduce and sift at his leisure.
The wretched man thus actually saw the planet twice--on August 4th and August 12th, 1846--without knowing it. If only he had had a map of the heavens containing telescopic stars down to the tenth magnitude, and if he had compared his observations with this map as they were made, the process would have been easy, and the discovery quick. But he had no such map. Nevertheless one was in existence: it had just been completed in that country of enlightened method and industry--Germany. Dr.
Bremiker had not, indeed, completed his great work--a chart of the whole zodiac down to stars of the tenth magnitude--but portions of it were completed, and the special region where the new planet was expected happened to be among the portions already just done. But in England this was not known.
Meanwhile, Mr. Adams wrote to the Astronomer-Royal several additional communications, making improvements in his theory, and giving what he considered nearer and nearer approximations for the place of the planet.
He also now answered quite satisfactorily, but too late, the question about the radius vector sent to him months before.
Let us return to Leverrier. This great man was likewise engaged in improving his theory and in considering how best the optical search could be conducted. Actuated, probably, by the knowledge that in such matters as cataloguing and mapping Germany was then, as now, far ahead of all the other nations of the world, he wrote in September (the same September as Sir John Herschel delivered his eloquent address at Southampton) to Berlin. Leverrier wrote, I say, to Dr. Galle, head of the Observatory at Berlin, saying to him, clearly and decidedly, that the new planet was now in or close to such and such a position, and that if he would point his telescope to that part of the heavens he would see it; and, moreover, that he would be able to tell it from a star by its having a sensible magnitude, or disk, instead of being a mere point.
Galle got the letter on the 23rd of September, 1846. That same evening he did point his telescope to the place Leverrier told him, and he saw the planet that very night. He recognized it first by its appearance. To his practised eye it did seem to have a small disk, and not quite the same aspect as an ordinary star. He then consulted Bremiker's great star chart, the part just engraved and finished, and sure enough on that chart there was no such star there. Undoubtedly it was the planet.
The news flashed over Europe at the maximum speed with which news could travel at that date (which was not very fast); and by the 1st of October Professor Challis and Mr. Adams heard it at Cambridge, and had the pleasure of knowing that they were forestalled, and that England was out of the race.
It was an unconscious race to all concerned, however. Those in France knew nothing of the search going on in England. Mr. Adams's papers had never been published; and very annoyed the French were when a claim was set up on his behalf to a share in this magnificent discovery.
Controversies and recriminations, excuses and justifications, followed; but the discussion has now settled down. All the world honours the bright genius and mathematical skill of Mr. Adams, and recognizes that he first solved the problem by calculation. All the world, too, perceives clearly the no less eminent mathematical talents of M.
Leverrier, but it recognizes in him something more than the mere mathematician--the man of energy, decision, and character.
LECTURE XVI
COMETS AND METEORS
We have now considered the solar system in several aspects, and we have pa.s.sed in review something of what is known about the stars. We have seen how each star is itself, in all probability, the centre of another and distinct solar system, the const.i.tuents of which are too dark and far off to be visible to us; nothing visible here but the central sun alone, and that only as a twinkling speck.
But between our solar system and these other suns--between each of these suns and all the rest--there exist vast empty s.p.a.ces, apparently devoid of matter.
We have now to ask, Are these s.p.a.ces really empty? Is there really nothing in s.p.a.ce but the nebulae, the suns, their planets, and their satellites? Are all the bodies in s.p.a.ce of this gigantic size? May there not be an infinitude of small bodies as well?
The answer to this question is in the affirmative. There appears to be no special size suited to the vastness of s.p.a.ce; we find, as a matter of fact, bodies of all manner of sizes, ranging by gradations from the most tremendous suns, like Sirius, down through ordinary suns to smaller ones, then to planets of all sizes, satellites still smaller, then the asteroids, till we come to the smallest satellite of Mars, only about ten miles in diameter, and weighing only some billion tons--the smallest of the regular bodies belonging to the solar system known.
But, besides all these, there are found to occur other ma.s.ses, not much bigger and some probably smaller, and these we call comets when we see them. Below these, again, we find ma.s.ses varying from a few tons in weight down to only a few pounds or ounces, and these when we see them, which is not often, we call meteors or shooting-stars; and to the size of these meteorites there would appear to be no limit: some may be literal grains of dust. There seems to be a regular gradation of size, therefore, ranging from Sirius to dust; and apparently we must regard all s.p.a.ce as full of these cosmic particles--stray fragments, as it were, perhaps of some older world, perhaps going to help to form a new one some day. As Kepler said, there are more ”comets” in the sky than fish in the sea. Not that they are at all crowded together, else they would make a cosmic haze. The transparency of s.p.a.ce shows that there must be an enormous proportion of clear s.p.a.ce between each, and they are probably much more concentrated near one of the big bodies than they are in interstellar s.p.a.ce.[30] Even during the furious hail of meteors in November 1866 it was estimated that their average distance apart in the thickest of the shower was 35 miles.
Consider the nature of a meteor or shooting-star. We ordinarily see them as a mere streak of light; sometimes they leave a luminous tail behind them; occasionally they appear as an actual fire-ball, accompanied by an explosion; sometimes, but very seldom, they are seen to drop, and may subsequently be dug up as a lump of iron or rock, showing signs of rough treatment by excoriation and heat. These last are the meteorites, or siderites, or aerolites, or bolides, of our museums. They are popularly spoken of as thunderbolts, though they have nothing whatever to do with atmospheric electricity.
[Ill.u.s.tration: FIG. 95.--Meteorite.]
They appear to be travelling rocky or metallic fragments which in their journey through s.p.a.ce are caught in the earth's atmosphere and instantaneously ignited by the friction. Far away in the depths of s.p.a.ce one of these bodies felt the attracting power of the sun, and began moving towards him. As it approached, its speed grew gradually quicker and quicker continually, until by the time it has approached to within the distance of the earth, it whizzes past with the velocity of twenty-six miles a second. The earth is moving on its own account nineteen miles every second. If the two bodies happened to be moving in opposite directions, the combined speed would be terrific; and the faintest trace of atmosphere, miles above the earth's surface, would exert a furious grinding action on the stone. A stream of particles would be torn off; if of iron, they would burn like a shower of filings from a firework, thus forming a trail; and the ma.s.s itself would be dissipated, shattered to fragments in an instant.
[Ill.u.s.tration: FIG. 96.--Meteor stream crossing field of telescope.]
[Ill.u.s.tration: FIG. 97.--Diagram of direction of earth's...o...b..tal motion, showing that after midnight, _i.e._ between midnight and noon, more asteroids are likely to be swept up by any locality than between noon and midnight. [From Sir R.S. Ball.]]
Even if the earth were moving laterally, the same thing would occur. But if earth and stone happened to be moving in the same direction, there would be only the differential velocity of seven miles a second; and though this is in all conscience great enough, yet there might be a chance for a residue of the nucleus to escape entire destruction, though it would be sc.r.a.ped, heated, and superficially molten by the friction; but so much of its speed would be rubbed out of it, that on striking the earth it might bury itself only a few feet or yards in the soil, so that it could be dug out. The number of those which thus reach the earth is comparatively infinitesimal. Nearly all get ground up and dissipated by the atmosphere; and fortunate it is for us that they are so. This bombardment of the exposed face of the moon must be something terrible.[31]
Thus, then, every shooting-star we see, and all the myriads that we do not and cannot see because they occur in the day-time, all these bright flashes or streaks, represent the death and burial of one of these flying stones. It had been careering on its own account through s.p.a.ce for untold ages, till it meets a planet. It cannot strike the actual body of the planet--the atmosphere is a sufficient screen; the tremendous friction reduces it to dust in an instant, and this dust then quietly and leisurely settles down on to the surface.