that is to say, more than a month later. We have, then, only to remember that the mean velocity of the earth in its orbit is 1,675,000 miles per day, and a very simple calculation will suffice to prove that the comet of six and three-quarter years period will be, at least during its apparition in 1832, always more than 49 millions of miles from the earth!'

Arago did well to reserve the question of future apparitions; for this same comet of 1832, forty years later, if it did not come into collision with the earth, must at least have grazed its surface.

Finally, before leaving these imaginary rencontres of comets and the end of the world, let us remark that the same chimerical fears were current in Europe in 1857, à propos of the predicted return of the comet of Charles V. This time the mystification came from Germany; according to a fantastic prediction the world was to be destroyed by fire, burnt by the terrible comet of June 13, in the year of grace 1857! The prediction at first related to the rencontre of an imaginary comet only; but afterwards the serious expectancy with which astronomers awaited the return of the comet of 1264 and 1556 suggested the idea of attributing the future catastrophe to the expected comet, although nothing in the elements of its orbit justified the probability of such a rencontre.*

In his notice upon the comet of 1832 Arago raises the following question: Can a comet come into collision with the earth or any other planet? By reasoning based upon the fact that the orbits of comets intersect the heavens in all directions, that they constantly traverse our solar system, and penetrate

*

Perhaps it had been ascertained that the comets of 1264 and 1556, in Lalande's table, approached the earth's orbit to about 0.08 of the distance of the earth from the sun; and that the earth and the comet passed on the same day (March 12, 1556) through the points of their nearest mutual approach-at 7,200,000 miles, however, from each other.

เ

to the interior of the orbits of the planets, even to the regions comprised between Mercury and the sun, he comes to the conclusion that it is not at all impossible for a comet to encounter the earth.' But, after having stated the possibility of the fact, he hastens to examine its probability. For this purpose he supposes a comet of which nothing more is known than that at its perihelion it will be nearer to the sun than we are ourselves, and that its diameter is equal to a fourth part of the earth. By the theory of probabilities Arago then finds that the odds are 281 millions to one against a rencontre with the earth. This probability, it is true, should be incrcased at least tenfold if, instead of our rencontre with the cometary nucleus, we were to substitute the entire nebulosity, the volume of which is comparatively much more considerable.

To illustrate the significance of the numerical results to which considerations of this kind lead, Arago continues: 'Let us for a moment suppose that if a comet were to strike the earth with its nucleus, it would annihilate the whole of the human race. The danger of death to which each individual, in that case, would be exposed from the apparition of an unknown comet would be exactly equal to the danger he would incur, supposing that in an urn containing a total number of 281 millions of balls there should be only one white ball, and that his condemnation to death were the inevitable consequence of this same white ball presenting itself at the first drawing. Anyone who will consent to use his reason, however attached to life he may be, will laugh at so trifling a danger; and yet the moment that a comet is announced, before it has been observed or its course determined, it is for each individual of our globe the white ball of the urn above-mentioned.'

These calculations are indisputably correct as regards their general application. But, when a particular comet is in question, whose elements are known, all considerations of probability

are out of place. Arago with reason calls attention to this fact in reference to the comet of 1832, and its apparition in that year. With respect to ulterior apparitions it is different. There is always some uncertainty; the orbit of the comet and its elements may be modified by the planetary perturbations, and the return of the comet to its node may take place so that both the earth and the comet may arrive, if not at the same point together, at least sufficiently near one another to cause the contact of some of their parts. We have already said that this is what probably took place in 1872, as regards the comet of 1832, during the night of November 27 The meeting was absolutely inoffensive.

SECTION III.

MECHANICAL AND PHYSICAL EFFECTS OF A COLLISION WITH

A COMET.

Opinions entertained by astronomers of the last century: Gregory, Maupertuis, Lambert-Calculations of Lalande; comets move too rapidly in the vicinity of the earth for the effects of their attraction to come into play-Opinion of LaplaceThe collision of a comet with the earth; its effect according to the mechanical theory of heat.

It is interesting to note the opinions formed by savants a century ago respecting the probable effect of a collision between a comet and the earth. Further on we shall speak of the theological romance invented by Whiston for the scientific explanation of the Deluge. According to Whiston the famous comet of 1680, after having, 4000 years ago, produced the universal deluge, is destined to accomplish the destruction of the world, and our globe will be ultimately set on fire by the same comet which had previously inundated it.

Whiston wrote at the end of the seventeenth century. In the middle of the eighteenth century theological speculations engaged but very slightly the attention of astronomers; but that a very exaggerated idea continued to prevail respecting the amount of injury which the proximity of a comet or its collision with the earth would be capable of producing is undoubted.

In 1742 Maupertuis, in his Lettre sur la Comète, writes as

follows: With their variety of movements it is clearly possible for a comet to encounter some planet or even our earth upon its way; and it cannot be doubted that terrible results would ensue. On the mere approach of these two bodies great changes would be effected in their movements, arising either from their mutual attraction or from the action of some fluid compressed between them. The least of these movements would suffice to change the position of the axis and the poles of the earth. That portion of the globe which had been previously near the equator would be situated, after such an event, near the poles, and that which had been near the poles would be situated near the equator.

'Some comets passing near the earth,' he observes elsewhere, 'might so alter its movement as to cause it to become a comet itself. Instead of pursuing its course, as it now does, in an uniform region, having a temperature suitable to man and the animals which inhabit it, the earth, exposed to the greatest vicissitudes, sometimes scorched at its perihelion, sometimes frozen by the cold of the remotest regions of the heavens, would be perpetually passing from one extreme to another, unless some comet should again come near enough to change its course and re-establish it in its original uniformity.'

Lastly, some large comet might, if Maupertuis is to be believed, divert the earth from its present orbit and subject it to its own attraction; in a word, make a satellite of our globe, which, henceforth compelled to follow the movements of the comet, would be exposed to the same vicissitudes as on the preceding hypothesis. 'A comet could in like manner deprive us of our moon, and if nothing more happened to us we should have no reason to complain. But the severest accident of all would be if a comet were to come in contact with the earth and break it into a thousand pieces. Both bodies would doubtless be destroyed; but from the fragments