meet the earth and strike against it. We know that the comet of 1680 approaches very near to the surface of the sun, so that, following Pingré's summary of Whiston's views, 'hardly can the mouth of a volcano vomiting forth lava liquified by the interior consuming heat give an idea of the fiery atmosphere of this comet. The air will then interpose no obstacle to the activity of the central fire; on the contrary, the inflamed particles with which our atmosphere will be charged, carried down by their own weight into the half-open bowels of the earth, will powerfully second the action of the central fire. This comet might even separate the moon from the earth, and affect the diurnal and annual motion of the earth by rendering both these movements equal, and by destroying besides the eccentricity of the terrestrial orbit, which would again become circular as before the deluge. Lastly, after the saints have reigned a thousand years upon the earth, itself regenerated by fire, and rendered habitable anew by the Divine will, a comet will again strike the earth, the terrestrial orbit will be excessively elongated, and the earth, once more a comet, will cease to be habitable.'

Such is the romance conceived by Whiston, a man of great erudition and science, but who shared the fault of his time in wishing to make his conceptions accord both with theology and astronomy. We are here only concerned with the scientific side of the question; and it is certain, and it was so a hundred years ago, that Whiston's theory is untenable. We will only notice two vital objections: in the first place, the enormous mass we are compelled to assign to the comet of 1680, and which no astronomer of our time would admit as probable; in the second place, even assuming such a mass, its action would be of so short duration, by reason, as we have seen, of the relative velocities of the comet and the earth, that the supposed effects would not have time to manifest themselves. But geologists, we believe, would have other objections to make to

an hypothesis which we have only recorded because it is celebrated in science; and because the part it assigns to comets is truly curious.

A last and capital objection is this: The discussion of the elements of the comet of 1680 made by Encke with more accurate data than Halley possessed, has entirely overthrown the supposed chronological coincidences with its anterior apparitions. According to the new elements, the period of the comet is not 170 years (Euler), nor 575 years (Halley), nor 5,864 years (Pingré), but 8,814 years.

SECTION VI.

PASSAGE OF THE EARTH THROUGH THE TAIL OF A COMET

IN 1861.

Possibility of our globe passing through the tail of a comet-Has such an event ever taken place?-The great comet of 1861-Relative positions of the earth and one of the two tails of that comet-Memoir of M. Liais and the observations of Mr. Hind.

THUS far, in treating of the possibility of a rencontre between a comet and the earth, we have more especially had in view the nucleus, or rather that portion of the comet's nebulosity which constitutes the coma. The effects of the rencontre have been studied on certain hypotheses respecting the mass and physical constitution of the comet whose nucleus we have supposed to be solid; this is far from certain, and, in any case, seems to be exceptional, as it is only in certain comets that the head is sufficiently condensed to exhibit a luminous nucleus.

A rencontre, of much greater probability, is that which would arise from the passage of the earth through the voluminous nebulosity of which the tail is formed. In all probability the masses of these appendages are all but inappreciable. Whatever opinion we may form of their nature, whether we regard them with Cardan and certain savants of our day as purely optical effects without material reality, or see in them the most tenuous portions of the atmosphere of the comet

projected by a repulsive force, it appears certain that they consist of quantities of matter of extremely slight mass, and of even less density. It would be ridiculous to speak of a shock or any other mechanical effect; but it is not altogether evident that the matter of a comet might not produce some perceptible modification of the atmosphere of our globe.

Before considering what would happen in the event of the earth passing through the tail of a comet, we are naturally led to inquire if such an event has ever actually occurred. Now, according to several contemporary astronomers, the earth was, in fact, on June 30, 1861, plunged for some time in the nebulosity forming the large tail of the great comet of that year. M. Valz, in giving the elements of the comet, has observed: 'It follows that the comet having passed its node on June 28, at 9.50 p.m., at the distance of 0.132 from the orbit of the earth, the latter being less than 2° in advance of the node, must have been situated within the nebulosity of the tail, which was itself in the plane of the ecliptic. M. Loewy, in the Bulletin de l'Observatoire of July 12, likewise observes: It is probable that about June 28, the earth touched the tail of the comet.' M. Pape, of Berlin, was of a different opinion, his calculations leading him to conclude that an interval of more than two millions of miles separated the tail of the comet from the earth; but, according to M. Valz, this arises from the German astronomer having estimated the apparent breadth of the tail at 3°, whereas he himself estimated it at 6°, and Father Secchi at as much as 8°. M. Le Verrier, in giving the elements calculated by M. Loewy and Mr. Hind, adds the following remark: Did the earth pass through the tail of the comet? This question, apparently so simple, is, in reality, very complex. The calculations are complicated, and the data fail to determine this point with certainty.'

เ

That the earth did pass through the tail of the comet was

the opinion of Mr. Hind from the very first. The following is an extract from the letter written on this subject by the English astronomer to the editor of the Times:

'Allow me to draw attention to a circumstance relating to the present comet, which escaped my notice when I sent a communication on the 3rd instant, but which is now possessing some interest. It appears not only possible, but even probable, that in the course of Sunday last the earth passed through the tail, at a distance of, perhaps, two-thirds of its length from the nucleus.

The head of the comet was in the ecliptic at 6 p.m. on June 28, distant from the earth's orbit 13,600,000 miles on the inside, its longitude, as seen from the sun, being 279° 1'. The earth, at this moment, was 2° 4′ behind that point, but would arrive there soon after 10 p.m. on Sunday last. The tail of a comet is seldom an exact prolongation of the radius vector, or line joining the nucleus with the sun; towards the extremity it is almost invariably curved; or, in other words, the matter composing it lags behind what would be its situation if it travelled with the same velocity as the nucleus. Judging from the amount of curvature on the 30th, and the direction of the comet's motion as indicated by my orbit already published, I think the earth would very probably encounter the tail in the early part of that day, or, at any rate, it was cer tainly in a region that had been swept over by the cometary matter shortly before.'

M. Liais, who observed the same comet in Brazil, speaks with still more certainty. He bases his assertions upon observations of his own, made before and after the perihelion passage, upon the breadth and direction of the tail of the comet, as well as upon the elements of the orbit calculated by M. Seeling. We shall not enter into the details of the calculation and the discussion given by this savant in l'Espace Céleste, but