comets we have seen came some of them at least from suns in a condition quite unlike that of our sun as he now is, or else that the suns which in the far-off ages expelled those comets have long since ceased to be suns. Thus we either look back to a distant time when the sidereal universe was full of younger suns ruling over systems still incomplete, or else the eye of reason reveals to us countless invisible orbs, which once were suns, still peopling the realms of space, but no longer affording light and heat and life to the schemes which circle round them. Or we' may adopt the inference, which perhaps some will consider more probable than either, that both views are just; in which case we must assume that even now, at this very time, there exist all orders of suns within the sidereal universe-suns still growing; suns ruling over schemes already formed around them; and, lastly, dead and usedup suns, waiting, as it were, for some future change, by which they will be restored to activity and usefulness.

But may we not reasonably apply these considerations to the minor system, of which the planets are the members? If within the star depths we recognise, at least with the eye of reason, these various states of being, may it not be that within the planetary scheme like varieties exist? We are too apt to consider that a dull uniformity pervades those regions which we have not explored; and though the study of our own domain has revealed an inexhaustible variety of nature, condition, and structure, we are unwilling to extend the lesson to the spaces around us.

We look on the stars as suns like our own, on the planets as orbs like our earth, on their satellites as moons like ours; and we regard the ring of Saturn with wonder, because we know of nothing wherewith to compare it. But may we not believe that the wonderful variety we recognise among terrestrial phenomena characterises the planetary scheme, and in a yet greater degree the amazing system of which our sun is but an unimportant member?

THE COMET FAMILIES OF THE GIANT

PLANETS.

I HAVE already spoken in a previous article of the comet called Biela's-and sometimes the Double Comet, or the Lost Comet, because it had first divided into two, and eventually vanished altogether away. I have now to consider the class of comets to which Biela's belongedthe comets of short period. In this class may be included all those which have been watched at two or three returns to our neighbourhood, so that Halley's comet, with its period of about seventy years, is included in the class, while, on the other hand, the periods of such comets range down to the short interval of three and one-third years, recognised in the case of Encke's comet. I do not propose, however, to enter into the history of any of these bodies. My special object is to discuss the relations they present in connection with the meteorsystems respecting which I have spoken in my last article.

It will be remembered that we were led to the conclusion that the great comets, as well as the great

See p. 124, 'The Lost Comet.'

meteoric masses which from time to time fall upon the earth, were originally expelled from the stars. Now it might seem at a first view reasonable to conclude that the small comets also, and the meteor-systems which follow in their track, had a similar origin. For in great volcanic explosions on our own earth large masses of rock and other matter are accompanied by flights of smaller missiles, while sometimes the expulsion of large masses ceases for a while, and showers of scoria and cinders are alone expelled. If we assume that the eruptions taking place in suns are like terrestrial eruptions in this particular respect, so that now large masses, and now flights of small masses are ejected, we should find the smaller comets and their meteor-trains as readily explained as the large comets and the great aerolites which probably follow in their track.

But there are certain difficulties in the way of this explanation.

The comets of short period follow orbits which on the one hand do not pass near to the sun, while on the other, being closed curves, they necessarily differ altogether in shape from the paths pursued by bodies arriving from interstellar space. These comets, then, cannot possibly have been expelled from the sun, for matter expelled from the sun would either pass away from him or return to him, moving very nearly in a straight course. If we assume that these comets were expelled from stars, we have to explain how it is that their paths are now so differently

shaped from those they would have had on first reaching our system after their journey through the interstellar depths.

The ordinary explanation of this circumstance is, that one or other of the giant planets-Jupiter, Saturn, Uranus, and Neptune--has disturbed one of these comets as it arrived after its interstellar journey, and has compelled it to take up the orbit in which it at present travels. If a comet were travelling towards the sun on such a course as to pass very close to Jupiter, it would be for a time chiefly under the influence of Jupiter's attraction, and its path might be so changed, and its velocity so reduced, that thereafter it would travel on a closed curve around the sun. And so with the other planets just named. Now, there is a circumstance in the movements of the comets of short period which accords well with this explanation. If a comet were at any time so near to the planet Jupiter, let us say, as to be treated in the way described, the future path of the comet would necessarily pass through the spot where the disturbance took place; and although in the course of many years the comet's path might be considerably changed, yet a part of the path would always pass somewhat near to the orbit of Jupiter. And in point of fact this peculiarity is recognised in a more or less marked degree with all the comets of short period. Their paths so cling, as it were, about the paths of the giant planets, that I long since gave to those thus related to Jupiter the title of Jupiter's comet