out any such excuse as in the case of Lexell's, viz., the preponderant attraction of some great planet. Can it have come into contact or exceedingly close approach to some asteroid as yet undiscovered; or, peradventure, plunged into, and got bewildered among, the ring of meteorolites, which astronomers more than suspect?"

Both these explanations seem at a first view available. Biela's comet had a course carrying it through the outskirts of the zone of minor planets; and there was nothing whatever to prevent the comet from coming into collision with one of these bodies, or else approaching so nearly as to be greatly disturbed, and to travel thereafter on a different orbit. But an objection exists which Sir J. Herschel does not seem to have noticed. When the comet retired in 1852 it consisted of two distinct comets, separated by an intervening space of about 1,250,000 miles. Now it would be a singular chance which should bring one of these comets into collision with a minor planet, or so near as to occasion an important disturbance. But supposing this to happen, then the fellow-comet, not travelling in the wake of the first, but side by side, would certainly have escaped. For it must be remembered, that although 1,250,000 miles is a very small distance indeed by comparison with the dimensions of the solar system, it is an enormous distance compared with the dimensions of the minor planets,-some of which have a surface not much greater than that of an English county. The minor planet occasioning the comet's disturbance would presumably be

the newly discovered minor planets are on the average much smaller than those first detected. Now, the earth herself would have no very marked influence on a comet or meteor passing her at a distance of 1,250,000 miles; for it is to be remembered, that the comet as well as the earth would have an enormously rapid motion, and the disturbing power of the earth would therefore only act for a short time. But a minor planet-even the largest of the family, would not have the twenty-thousandth part of the earth's power to disturb a passing comet. At a distance of 200,000 miles, a comet would pass such an asteroid without any marked disturbance of its motions.

Of course it is not absolutely impossible that one of the comets of the pair should have been encountered by one minor planet, and the other by another; but the improbability against such a contingency is so great that we need scarcely entertain the idea even as a bare possibility.

We are left then to the supposition that the comet was destroyed or dissipated by meteoric streams. It is at once seen that this theory is at least more consistent with observed facts than the other. The comet had been seen to divide into two parts in a portion of the solar system, where certainly no bodies but meteorites can be supposed

* It is probable that the largest of the minor planets-Vesta-has a diameter of rather more than 200 miles, or at the outside say 260 miles -the thirtieth part of the earth's diameter. Thus, assuming Vesta to have the same density as the earth (whereas, being smaller, she probably is very much less compressed), we get for her mass (or, which is the same thing, her attractive power) the 27,000th part of the earth'sobtaining the number 27,000 by multiplying 30 twice into itself.

to travel. It seems reasonable to suppose, that on that occasion the head of the comet had come right upon some group of meteors, and so had divided as a stream of water divides against a rock. Assuming this, we find reason for believing that the track of this comet crosses a rich meteorregion. The particular group which had caused the division of the comet would of course pass away, and would not probably come again in the comet's way for many years or even centuries. But another group belonging to the same system might in its turn encounter the comet, and complete the process of dissipation which the former had commenced. On this theory, the distance between the companion comets would introduce no difficulty. For not only is it quite a common circumstance for meteoric systems to have a range of several millions of miles, but -a much more important consideration-both the comets would be bound to return to the scene of the former encounter. It was there that each had been sent off on a new track; but each new track started from there, and therefore each new track must pass through there.

*

So that it seems far from improbable that, if the comets could have been watched during their return in 1859, they would have been seen to travel onwards towards the place where they had originally separated; as they approached that place, it would have been perceived that they drew nearer together, though they would not reach that point at the same moment; † and then each in turn would have

* See the paper on meteors in the "Expanse of Heaven."

+ Of course in an article intended like the present for general reading,

appeared to grow more and more diffuse as the encounter with the meteor-group proceeded, until first one and then the other would have vanished altogether from view.

It may be asked, whether any circumstances in the history of comets seem to show that comets really are exposed to dissipation in this way. To this the reply is, that although Biela's is the only comet which has been seen to divide into parts in modern times, or under telescopic scrutiny, yet history records more than one instance of a similar kind,—and that too in the case of distinguished comets, not mere telescopic light-clouds such as Biela's. The following passage from Grant's noble work, "The History of Physical Astronomy," gives nearly all that is known on this point, though some Chinese records might be added did space permit:-"Seneca relates that Ephorus, an ancient Greek author, makes mention of a comet which, before vanishing, was seen to divide itself into two distinct bodies. The Roman philosopher appears to doubt the possibility of such a fact; but Kepler, with characteristic sagacity, has remarked that its actual occurrence was exceedingly probable. The latter astronomer further remarked, that there were some grounds for supposing that two comets, which appeared in the same region of the

it is not possible to enter at length into all the considerations which have to be attended to in an exact inquiry into the motions of two comets after separation. It will be sufficient to point out that, unless the collision which caused the separation left the velocity of each exactly equal-a wholly unlikely supposition-they would return to the scene of collision at different epochs. The increased distance between them in 1852 showed that this was actually the case.

heavens in the year 1618 were the fragments of a comet that had experienced a similar dissolution. Hevelius states, that Cysatus perceived in the head of the great comet of 1618 unequivocal symptoms of a breaking up of the body into distinct fragments. The comet, when first seen in the month of November, appeared like a round mass of concentrated light. On the 8th of December it. seemed to be divided into several parts. On the 20th of the same month it resembled a multitude of small stars. Hevelius states, that he himself witnessed a similar appearance in the head of the comet of 1661."

It is, of course, always possible that the destruction or dissipation of a comet may be due, not to any collision, but to that action (whatever may be its nature) by which the sun seems, after rousing and disturbing the matter of a comet's head, to repel a part of this matter in such sort as to form a tail, or two or more tails. Indeed, it is worthy of notice that before its division into two comets, Biela's comet had shown two distinct tail-like appendages; and possibly, if the comet could have been constantly watched it would have been found that these two appendages resolved themselves eventually into the two tails of two distinct comets.

Professor Grant adopts this view of the matter. He says, "it is impossible to doubt that the division of Biela's comet arose from the divellent action of the sun, whatever may have been the mode of operation." But I must admit, that I find it quite possible to doubt whether this is indeed