Page:Popular Science Monthly Volume 16.djvu/265

Rh the solid form is probably not many inches in diameter (I am speaking, be it remembered, of the meteors producing displays of ordinary shooting stars or falling stars, not of those masses which thrust their way through the upper regions of the air, and, exploding, cast their fragments often over many square miles of the earth's surface). It will be understood how small is the chance that a flight of bodies so minute compared with the average space occupied by each could cause the dispersion of a mass so rare, and therefore so free to pass through a meteor-flight without disintegration or disturbance, as a comet.

How Biela's comet came actually to be divided into two distinct bodies, and later to be so far dissipated as to be no longer visible even in the most powerful telescopes and under the most favorable circumstances, will probably be understood when we know the nature of those processes of repulsion which lead to the formation of comets' tails. For our present purpose it is only necessary to observe that these processes of repulsion do most obviously carry away parts of the substance of a comet's head to enormous distances, and that, in some way, Biela's comet was divided, even as it were under the eyes of astronomers, into two distinct comets; for we thus learn to recognize the further disintegration of the comet as part of a process undoubtedly commenced in 1846 and undoubtedly competent to effect the dissipation of the comet's substance. As the comet was searched for in vain in 1866 in the region which unquestionably it would have traversed had it remained unchanged, there can be no reason for doubting that it had thus been thoroughly dissipated and disintegrated. If anything could have made this more certain, it would have been the circumstance that in 1872, also, the comet was searched for in vain. Remembering that the observations made during the first few weeks after the comet's discovery in 1826 gave astronomers such a mastery over its motions that they could successfully predict its return in 1832, and show precisely where it would appear, nay, even calculate back its path and recognize its identity with a comet discovered by Montaigne in 1772, and rediscovered (though not recognized as the same) by Pons in 1805, it is obvious that in 1866, after several carefully observed returns and nearly a century after its first discovery, the comet's motions must have been much more thoroughly understood. It would have been much more easily detected that year than in 1846 and 1852, even as Halley's comet was much more easily detected at its return in 1835 than at its return in 1759.

If the comet had been like most of its fellows, astronomers must have given up all idea of obtaining further information respecting it. But in one important respect it differed from them. It is one of the few known comets whose paths cross, or at least pass very close to, the track of the earth. Already in 1832 attention had been called to this circumstance. Indeed, fears had been excited among those unfamiliar with astronomical relations by the announcement that the