Returning to Encke's comet, we have to notice yet another important discovery which was effected by its means. The comet passed so near to Mercury in 1835 as to enable astronomers to form a much more satisfactory estimate of this planet's mass than had hitherto been obtained. It was found that the mass of Mercury had been largely over-estimated.
No very long interval passed after the discovery of Encke's comet before another comet of short period was detected. M. Pons, who had discovered Encke's comet, it will be remembered, in 1818, observed a faint nebulous object on June 12, 1819. This object turned out to be a comet; and in this case, as in the former, Encke calculated the stranger's orbit and period. He found that it moves in an ellipse which extends slightly beyond the orbit of Jupiter, and that it has a period of about five and a half years. This object was not seen again, however, until the year 1858, when M. Winnecke discovered it, and at first supposed it to be a new comet. Calculation soon showed the identity of the two objects, and confirmed the results which had been obtained by Encke in 1819.
The next comet of short period was discovered by M. Biela in 1826. Perhaps nothing in the whole history of cometic observation is more surprising than what has been recorded of this singular object. We must premise that the comet had been seen in March 1772, and again in November 1805. But it was not until its re-discovery in 1826 that its orbit and period were computed. An ellipse of moderate eccentricity, extending beyond the orbit of Jupiter, was assigned as the comet's orbit-the period of revolution being about six and a half years. The orbit was found to pass within about twenty thousand miles of the earth's orbit; and at the first return of the comet (in 1832), some alarm was experienced lest the near approach of the two bodies should lead to mischief of some sort. The comet returned again in 1839 and 1845. It was at the last-mentioned return that a singular phenomenon occurred, which is nearly unique in the history of comets. On the 19th of December 1845, Hind noticed a certain protuberance on the comet's northern edge. Ten days later, observers in North America noticed that the comet had separated into two distinct comets, similar in form, and each having a nucleus, a coma, and a tail. European observers did not recognise the bi-partition of the comet until the middle of January 1846. The new and smaller comet appears to have sprung into existence from the protuberance observed by Hind, since this object moved towards the north of the other. After a while, the new comet became the brighter, but, shortly after, it resumed its original relative brilliancy. Lieutenant Maury noticed, on one occasion, a faint ' bridge-like connection' between the two comets. The distance between them gradually increased, until first the new comet, and then the old one, had passed out of view. In 1852, Biela's comet was again seen, and the
Padre Secchi, at Rome, detected a faint comet preceding it. If, as is probable, this faint comet is the companion, we may assume that the two bodies are permanently separated.
At the two next returns the comet was not seen, and much interest was felt by astronomers respecting the anticipated return in January 1866. It was searched for systematically at the principal European observatories. In fact, so closely did Father Secchi examine the calculated track of the comet, that he detected several new nebulae in that region. But the comet itself was not found. Astronomers are unable to assign any satisfactory reasons for its disappearance. It is known to have traversed the zone of the November meteors where that zone is richest-our readers will remember the display of shooting-stars in 1866-and Sir J. Herschel surmises that it may have been destroyed in the encounter. Possibly this may be the true solution of the difficulty; or, it may be that the comet was merely dispersed for a while during the passage of the meteor-zone, and may yet gather itself together and become visible to astronomers.1
We pass over three or four comets belonging to this class which present no special features of interest, to come to an object which has recently been rediscovered, and will continue visible (in good telescopes) for several weeks. On February £6, 1846, M. Brorsen discovered a telescopic comet, whose motions soon showed it to belong to the class of objects we are now dealing with. It was found to have an orbit of moderate eccentricity, extending just beyond Jupiter's orbit, and a period of about five and a half years. It was not seen at its next return to perihelion; but was rediscovered by M. Bruhns on March 18, 1857. In 1862, it again escaped undetected; but at its present return, it has been rediscovered (by three observers simultaneously), and it is now being carefully tracked across the northern skies.
1 The return of this comet in 1872 was eagerly looked for by astronomers. But the comet was not seen. On November 27, 1872, there was a fine display of meteors, as the earth passed through the comet's track, and afterwards a cometic object was seen in the direction towards which the meteors had been travelling. But this was not Biela's comet, which, indeed, must have passed that place nearly twelve weeks earlier. Indeed, some doubt exists whether the object was travelling in the track either of the comet or of the meteors.
In all, there have been recognised thirteen comets of short period - that is, having periods of less than seven years. Amongst these are included several which have only been seen once, and some which are known to have been subjected to such disturbance as no longer to travel in orbits of short period. Of these thirteen comets, no less than ten have the aphelia of their orbits just beyond the orbit of the planet Jupiter; two have their aphelia just within Jupiter's orbit; and Encke's comet alone has its aphelion at a safe distance from that orbit. It appears to us that the peculiarity thus exhibited is not without meaning. Remembering the history of Lexell's comet, we seem to find a satisfactory explanation of the peculiarity. We have seen how Lexell's comet was first introduced into the system of short-period comets by the giant planet Jupiter, and then summarily dismissed. So long as the comet remained within that system, the aphelion of its orbit lay just beyond the orbit of Jupiter, and this would be the case with any comet introduced in a similar manner. But for the coincidence which led to its expulsion, Lexell's comet would have continued to revolve as a short-period comet. It seems also clear, that in the course of many ages, its period and orbit would have grown gradually smaller, through the operation of the same cause (whatever that may be) which is now reducing the period and orbit of Encke's comet. At length it must have attained a path safe within the orbit of the great disturbing planet. In the list of short-period comets, then, we seem to see illustrations of the successive stages through which Lexell's comet would have passed in attaining the sort of orbit in which Encke's comet is now moving. And it seems permissible to assume that all the short-period comets have been introduced to their present position within the solar system by the same cause which led to the temporary appearance of Lexell's comet as a comet of short period-that is, by the attractive energy of the planet Jupiter.