Fig. 3.—Halley's Comet of 1835.
This, however, is as nothing compared with the mighty range in space and the enormous period of the orbit of the great comet of the year 1811 (fig. 4). This comet is, on the whole, the most remarkable ever known. It was visible for nearly seventeen months, and though it did not approach the sun within 100,000,000 miles, and was therefore not subject to that violence of action which has caused enormous tails to be thrown out from comets which have come within a few million miles of him, or even within less than a quarter of his own diameter, it flourished forth a tail 120,000,000 of miles in length. Its orbit has, according to the calculations of the astronomer Argelander, a space exceeding the earth's distance from the sun 211 times, and thus surpassing even the mighty distance of Neptune fully seven times. It occupies in circuiting this mighty path no less than 3065 of our years (with a possible error either way of about forty-three years). So that, according to Bible chronology, this comet's last appearance probably occurred during the rule of the judge Tola, son of Puah, son of Dodo, over the children of Israel, though it may have occurred during the rule of his predecessor Abimelech, or during that of his successor Jair.[1] During one half of the enormous interval between that time and 1811 the comet was rushing outwards into space, [25][26] [27] reaching the remotest part of its path somewhere about the year 278 (A.D.), and from that time to 1811 it was on its return journey. It is strange to think, however, that though the remotest part of its path lay 211 times farther from the sun than the earth's orbit, yet even this mighty path, requiring more than 3000 years for a single circuit, cannot be said to have carried the comet into the star-depths. If the earth were to shift its position by the same enormous amount the nearest fixed star would have its apparent position changed only by about an eighth part of the apparent diameter of the sun or moon, or by about one-quarter of the distance separating the middle star of the Bear's tail from its close companion.
Fig. 4.—Comet of 1811.
But this fact of itself is most strikingly suggestive of the vast distance of the stars. For consider what it means. Imagine the middle star of the Bear's tail to be the really nearest of all the stars instead of lying probably twenty or thirty times farther away. Conceive a comet belonging to that sun after making its nearest approach to it to travel away upon an orbit requiring 3000 years for each circuit. Then (supposing that star equal to our sun in mass), the comet, though rushing away from its sun with inconceivable velocity during 1500 years, would, at the end of that vast period, seem to be no farther away than one-fourth of the distance separating the sun from its near companion. Look at the middle star of the Bear's tail on any clear night, and on its small satellite, remembering this fact, and the awful immensity of the star depths are strongly impressed upon the mind. But the observer must not fail to remember that the star really is many times more remote than we have here for a moment supposed, and that such a comet's range of travel would be proportionately reduced. Moreover, many among the stars are, doubtless, hundreds, even thousands, of times still farther away.
Fig. 5—Six-tailed Comet of 1744.
Let us turn lastly to the amazing comet of the year 1744, pictured, at the time, as shown in fig. 5 (though probably the drawing is greatly exaggerated). We find that though it had the longest period of any which has ever been assigned to a comet as the result of actual mathematical calculation, yet its range in space would scarcely suffice to change the position of the stars in such sort that the aspect of the familiar constellations would be materially altered. Euler, the eminent mathematician, calculated for this comet a period of 122,683 years, which would correspond, I find, to a distance of recession equal to 2469 times the distance of the earth from the sun, or about eighty times the distance of Neptune. Yet this is but little more than twelve times the greatest distance of the comet of 1811. Probably the actual range of such an orbit from the middle star of the Bear's tail would be equal in appearance to the range described above on the supposition that the star is no farther from us than the nearest known star (Alpha Centauri). That is, such a comet, if it could be seen and watched during a period of about 122,000 years, would seem to recede from the star to a distance equal to about one-fourth the space separating it from its close companion, and then to return to the point of nearest approach to its ruling sun.
Such are the immensities of star-strewn space! The journey of a comet receding from the sun with inconceivable velocity during hundreds of thousands of years carries it but so small a distance from him compared with the distance of the nearest star as scarcely to change the appearance of the celestial landscape; and yet the distances separating the sun from the nearest of his fellow suns are but as hair-breadths to leagues when compared with the proportions of the scheme of suns to which he belongs. These distances, though so mighty that by comparison with them the inconceivable dimensions of our own earth sink into utter nothingness, do not bring us even to the threshold of the outermost court of that region of space to which the scrutiny of our telescopes extends. Yet the whole of that region is but an atom in the infinity of space.
III.
OF THE INFINITELY MINUTE.
When I speak of the infinitely minute, I use the word infinitely not in its absolute sense, but relatively. Actual infinity of minuteness is as utterly beyond our conceptions as actual infinity of vastness. But we may speak of what is very much less than the least object of which our senses can make us directly conscious as for us infinitely minute. Among the greatest wonders science has to deal with are those relating to bodies and movements thus beyond the direct ken of our senses. There is a universe within the universe which our senses reveal to us—a universe whose structure is so fine that the minutest particle which the microscope can reveal to us is, by comparison, like one of the suns which people our universe compared with the unseen particles constituting matter.
It is a strange thought that the objects constituting our universe, so long regarded by man as the only universe, are in a sense pervaded by the materials of an utterly different universe—which yet is as essential to our very existence as what we commonly call matter. We cannot live without light and heat, for instance, and again, light and heat affect matter as we know it; but they thus exist and affect such matter by means only of a form of matter unlike any which we can conceive. It is certain that if absolute vacancy separated our earth from the sun, even by the narrowest imaginable gap, his heat and light could never reach us. They could no more pass that vacant space than the wave-motion of water can cross a space where water itself is wanting. It is because of relations such as these that it has been said, and justly, that matter is the less important half of the material constituting the physical universe.
Our knowledge of this universe within our universe has been obtained within comparatively recent years. Men were unwilling or at least they spoke and thought as if they were unwilling, to believe that the universe of matter which they had so long recognised was dependent on another universe for its chief if not all its properties. They regarded heat as some sort of