A first difficulty arises from the extremes of temperature to which comets are subjected at their aphelia and perihelia. 'How are we to conceive,' he observes, that beings can exist in an abode which is subjected to the utmost extremes of heat and cold? The comet which appeared in 1759 (that of Halley) and which returns the quickest of all those whose periods are known undergoes a winter 70 years long. But there is even a greater extreme of heat.' Although Lambert objects to Newton's calculation as to the heat to which the comet of 1680 must have been subjected during its perihelion passage, still he is obliged to admit that on the 8th December, 1680, 'the comet being one hundred and sixty times nearer to the sun than we are ourselves, must have been subjected to a degree of heat twenty-five thousand six hundred times as great as we are. Whether this comet was of a more compact substance than our globe, or was protected in some other way, it made its perihelion passage in safety, and we may suppose all its inhabitants also passed safely. No doubt they would have to be of a more vigorous temperament and of a constitution very different from our own. But why should all living beings necessarily be constituted like ourselves? Is it not infinitely more probable that amongst the different globes of the universe a variety of organizations exist, adapted to the wants of the people who inhabit them, and fitting them for the places in which they dwell, and the temperatures to which they will be subjected? Have we not in like manner abandoned the prejudice which for a length of time caused the torrid and frigid zones to be regarded as uninhabitable? Is man the only inhabitant of the earth itself? And if we had never seen either bird or fish, should we not believe that the air and water were uninhabitable? Are we sure that fire has not its invisible inhabitants, whose bodies, made of asbestos, are impenetrable to flame? Let us admit that the nature of the beings who

inhabit comets is unknown to us; but let us not deny their existence, and still less the possibility of it.'

Thus regarded as a matter of pure hypothesis, it is plain that the question of the habitability of comets may always be answered in the affirmative. But we must not forget that at the time when Lambert wrote, comets were regarded as solid bodies enveloped by a considerable atmosphere, and the tendency to assimilate them to the planets was general; add to this a few vague ideas upon the subject of final causes such as Lambert held-and it was natural to people all the stars of heaven, and even the sun himself with inhabitants.

Andrew Oliver published nearly about the same date (1772) an Essay upon Comets, wherein he seeks to explain the formation of tails by a mutual repulsion of electric origin, between the atmospheres of the sun and the comet: he devotes the second part of his curious work to showing that the tails of comets are probably intended to render their bodies habitable worlds. The enormous variations of temperature to which a comet is subjected in passing from one extremity of its orbit to the other, are exactly or at least suitably compensated by variations in the density of its atmosphere. This, together with the movements due to the action of the sun and the supposed velocity of rotation, prevents the extremes of heat and cold from becoming intolerable. becoming intolerable. At the aphelion both its atmosphere and tail are condensed about the comet, and the air is in a state of perfect calm. In proportion as it approaches perihelion, the atmosphere becomes rarified, the equilibrium is constantly broken, and currents of fresh air temper the extreme ardour of the solar rays.

These, as we see, are but physical romances composed by the partisans of a preconceived idea of the habitability of these bodies. Neither Fontenelle, nor Lambert, nor Andrew Oliver would probably write at the present day as they did a hundred

or a hundred and fifty years ago. And for this two reasons may be assigned, the one philosophical, the other scientific. In the first place the à priori has by common consent been banished from science, which leaves to metaphysics the task of supporting theses by arguments based upon ideas such as that of final causes. We no longer ask for example how comets must be constituted to permit the existence of living beings, which Providence could not have withheld from bodies so numerous and important. But we seek by the study of observed facts and by the discussion of the probable physical consequences which must follow from these facts to form an approximate idea of the conditions-physical, luminous, calorific, and chemical— of known comets. And should he then enter upon the question of the habitability of these bodies, we do not consider it in the absolute and unconditional manner in which it was entertained by Lambert. We merely compare the probable conditions as determined by observation with those which seem to be compatible on the surface of the terrestrial globe with the existence of organized living beings. In short, there has been a total change of method.

A second reason which would have brought about a change in the opinions of the eminent savants whose theories we have just quoted, is that within the last hundred years—as we have seen in detail the physical and even chemical constitution of comets has been carefully studied. We no longer assimilate them to the planets except as regards their movement of translation. Everything leads us to believe that the agglomerations of which they are composed are in a rudimentary state analogous to the rudis indigestaque moles of chaos. The incessant transformations which take place in their nuclei, their atmospheres and their tails indicate an equilibrium eminently unstable, and which would be very difficult to reconcile with the known conditions of life.

After this, let all who please picture to themselves the comet which has lately paid us so brief a visit [July, 1874], peopled with astronomers such as those of whom Lambert* speaks. We will not cavil with them; we do not fight with shadows.

* 'I like to picture to myself,' he says, 'these globes, voyaging in space, and peopled with astronomers, who are there on purpose to contemplate nature on a grand scale, as we contemplate it on a small scale. From their moving observatory, as it is wafted from sun to sun, they see all things pass successively before their view, and can determine the positions and motions of all the stars, measure the orbits of the comets and the planets which glide by them, see how the particular laws develop into general laws, and know, in a word "the details of the universe." In truth, I picture to myself that astronomy must be for the inhabitants of such a comet a terribly complicated science. But doubtless their intelligence is proportional to the difficulties.'

SECTION II.

WHAT WOULD BECOME OF THE EARTH IF A COMET WERE TO MAKE IT ITS SATELLITE ?

Conditions of temperature to which the earth would be subjected if it were compelled by a comet to describe the same orbit as the latter-The comets of Halley, and of 1680, examined from this point of view-Extremes of heat and cold: opinion of Arago impossibility of living beings resisting such changes.

ARAGO has examined, in an indirect manner, the question of the habitability of comets; that is to say, he has considered how far the enormous distances through which a body passes in describing a very eccentric orbit around the sun, such as that of a cometary orbit, are compatible with the existence of inhabitants similar to man. Could the earth, he enquires, ever become the satellite of a comet, and, if so, what would be the fate of its inhabitants?

Arago, basing his reasoning upon the comparative smallness of the masses of comets, regards the transformation of the earth into the satellite of a comet, as an event within the bounds of possibility, but which is very improbable,' an opinion no one at the present day will be inclined to dispute. He next supposes our earth successively made tributary to the comet of Halley and to that of 1680, and proceeds to consider the conditions of temperature to which our globe would be subjected whilst travelling in company with each.