regions as the planets, projecting its substance far beyond its own sphere of attraction, a comet can scarcely fail to abandon fragments of its tail, which the mass of the earth, for example, may afterwards appropriate.

These fragments, it is true, by the common consent of all astronomers, are but trifling, materially speaking, and their total weight is only a very insignificant fraction of that of our atmosphere; but might not the continued introduction of these particles into the air we breathe become in the course of time a source of sickness and death to the living beings inhaling them? Might not certain kinds of epidemics be thus explained? This is a question which we scarcely have the means of answering. If the tails of comets are formed of matter so attenuated, so little coherent, it is reasonable to suppose that they may be attracted to the earth and become an integral part of it. But how are we to suppose that they descend into the depths of this envelope? At the utmost they could only float at the extreme limit of the atmosphere, and the supposed gas of which they are composed would not in any way mix with the gases of the air which human beings and animals respire.* Suppose these gaseous particles are endowed with a peculiar chemical activity, and that their contact with oxygen or nitrogen determines the formation of dense and poisonous precipitates: even then these particles of a matter so prodigiously dilated in the beginning, would contribute when condensed but an infinitesimal quantity to the air we breathe, and, unless we have faith in the homœopathic doctrine, need inspire us with no alarm.

Appeal has been made to the facts. As writers who believed in the supernatural and providential influence of comets have

[* *If we suppose the tail of the comet to consist of a gas, it would mix with the other gases of the atmosphere in accordance with the known law of diffusion. If a light gas be placed upon a heavy gas, the latter will not remain floating as it were upon the former, but after a time the two will become completely mixed. --ED.]

collected all historical details attending upon each apparition which might seem to bear testimony in favour of their superstition, so have the advocates of a connexion between epidemics and comets collected a number of supposed accordances. Arago quotes from Dr. T. Forster, who had expended a large amount of erudition in forming a catalogue of so-called cometary influences.

'Mr. Forster has,' he observes, 'so extended the circle of supposed cometary actions that there is scarcely a phenomenon in nature that may not be ascribed to cometary influence. Cold and warm seasons, earthquakes, volcanic eruptions, great hail-storms, abundant snows, heavy rains, floods, droughts, famines, thick clouds of flies or locusts, the plague, dysentery, epizootic diseases, all are recorded by Mr. Forster with reference to each cometary apparition, no matter what the continent, kingdom, town, or village subjected to the ravages of the plague, famine, &c.' For example, the date of the comet of 1668 corresponds to the remark that all the cats in Westphalia were sick; that of 1746 to the earthquake in Peru which destroyed Lima and Callao; other dates, again, correspond to the fall of an aerolite, to the passage of numerous flocks of pigeons, &c. This is truly an absurd enumeration. It recalls to mind Bayle's letter, and his parallel of the lady and the carriages in the Rue St. Honoré. But the whole is too puerile to need refutation.

Of meteorological phenomena attributed to comets because their causes have remained unknown, mention must be made of dry fogs, such as those of 1783, 1822, 1831, and 1834.

The appearance of this singular phenomenon, and the circumstances which, in 1783 more particularly, accompanied its long duration (it was visible more than a month), explain to a certain extent this hypothesis. Hygrometrically, this fog had not the qualities of an ordinary fog: it was not wetting.

De Saussure's hygrometer marked only 57°; the general colour of the air was that of a dull, dirty blue; distant objects were blue, or surrounded by mist, and at the distance of a league were undistinguishable. The sun, red, without brilliancy, and obscured by mist, both at his rising and setting, could be steadfastly regarded at noonday. A singular circumstance mentioned by Arago is that the dry fog of 1783 appeared to possess a certain phosphoric property, a light of its own. 'I find at least in the accounts of some observers,' he remarks, 'that it diffused, even at midnight, a light which they compare to that of the moon at its full, and which sufficed to make objects distinctly visible at a distance of more than 200 yards.'

Was the earth plunged in the tail of a comet, or had it met with the fragment of a cometary appendage abandoned in space? But why, then, was not the comet itself visible? Meteorologists (Kämtz) still continue to rank dry fogs amongst problematical phenomena; nevertheless, it was remarked that in 1783, at the two extremities of Europe, violent physical commotions took place; continued earthquakes in Calabria, and a volcanic eruption in Iceland. Could the dust and ashes projected to a distance and scattered far and wide have been the cause of the phenomenon?

Dry fogs are common in Holland, and also in the west and north of Germany. Finke tells us that they are due to the smoke produced by the combustion of the turf-beds. In 1834 the drought did in fact cause numerous fires in the forests and turf-beds of Prussia, Silesia, Sweden, and Russia.

Franklin assumed, in order to explain the dry fog of 1783, the diffusion of volcanic cinders and emanations. He likewise supposes and this hypothesis is closely allied to that of the earth's immersion in the train of a comet-that an immense bolide might penetrate into our atmosphere, be there imperfectly consumed, and diffuse torrents of smoke or light

ashes. We shall presently see that certain rains of dust can be explained in a similar manner. Many savants admit as a very probable fact that matter of extra-terrestrial origin may penetrate into the atmosphere and fall to the ground, and perhaps modify the constitution of the gaseous envelope in which we live.

SECTION IV.

CHEMICAL INFLUENCES OF COMETS.

Introduction of poisonous vapours into the terrestrial atmosphere-The end of the world and the imaginary comet of Edgar Poe; Conversation of Eiros and Charmion -Poetry and Science; impossibilities and contradictions.

We now come to that other cometary influence which we have already alluded to, an influence capable of changing the air we breathe by the introduction of foreign effluvia.

Nothing within the range of fact and observation, up to the present time, affords ground for belief in such an influence. But this hypothesis has had the fortune to be presented in a striking and practical form by a modern writer of powerful imagination. The American poet Edgar Poe, whose Extraordinary Histories are known to everyone, has placed in the mouth of a being who has suffered death, an account of the destruction of the world by the near approach of a comet. We subjoin the principal portion of this wonderful dream, in which Eiros relates to Charmion the circumstances which put an end to the world.

'The individual calamity was, as you say, entirely unanticipated, but analogous misfortunes had been long a subject of discussion with astronomers. I need scarce tell you, my friend, that, even when you left us, men had agreed to understand those passages in the most holy writings which speak of