SECTION II.

DO COMETS EXERCISE ANY INFLUENCE UPON THE SEASONS?

Study of the question by Arago-The calorific action of comets upon the earth

appears to be inappreciable-Comparison of the meteorological statistics of various years in which comets did and did not appear-The meteorological influence of a comet is not yet proved by any authentic fact.

WE have already said how general a consternation was created in 1832 by the announcement that Biela's comet would pass within a very short distance of the orbit of the earth. Arago made it the occasion of one of those brilliant and interesting notices in which he endeavoured to destroy existing prejudices, and to render the simple truths of astronomy better and more generally understood. The heading of one section of this notice was

Will the future Comet modify in any appreciable degree the Course of the Seasons of the year 1832?'

To this question he replies in the following terms :

'The above title will doubtless call to mind the beautiful comet of 1811, the high temperature of that year, the abundant harvest following, and, above all, the excellent quality of the comet wine. I am therefore well aware that I shall have to contend with many prejudices in order to establish that neither the comet of 1811, nor any other known comet, has ever occasicned the smallest change in the seasons. This opinion is founded upon a careful examination and attentive discussion

of all the elements of the problem, whilst the opposite idea, however widely spread it may be, has no foundation whatever in fact.

It is said that comets heat our globe by their presence. Be this as it may, nothing is easier to verify. Is not the thermometer consulted many times a day in all the observatories throughout Europe? Is not an exact record kept of all the comets which appear?'

Thereupon Arago proceeds to tabulate the mean temperatures of the years between 1803 and 1831, at the same time placing by the side of them the numbers of comets observed, together with any peculiarities exhibited by them which could exercise an influence upon temperature. He has since extended this instructive table from 1735 to 1853, and proves without difficulty that no law connects the variations of mean temperature with the apparition of comets, and that years fruitful in apparitions, such as those of 1808, 1819, 1846, for example, have been marked by temperatures lower, or hardly equal to those of years in which few or no comets have been seen.

The whole of the sixty-nine comet years give a mean temperature of 51°.46 Fahr.; twenty-seven years without comets give a mean of 50°.94 Fahr. The difference of the half degree Fahrenheit, Arago explains by the fact that years without comets are most frequently cloudy; the prevalence of cloud simply concealing the comet or comets from observation. This difference becomes almost inappreciable when he compares the mean temperatures of the thirty years, in each of which only one comet appeared, and the thirty-nine years, in each of which two or several comets appeared. The difference in this case is no more than four-hundredths of a degree Fahrenheit, a quantity absolutely insignificant.

Other tables, founded upon analogous data, further establish 'that very low temperatures have frequently taken place during

the apparition of comets, and very high temperatures at epochs when none of these bodies have been visible.'

Returning, then, to the comet of 1811, Arago considers how far it was possible for the brilliant train of that body to exercise an influence upon our globe? It was 102,000,000 of miles in length, it is true; but then it was not strictly directed towards the earth, and the comet at its least distance from our globe was separated by 117,000,000 of miles. Moreover, we are now assured of the extreme tenuity of these cometary appendages, and the insignificant amount of heat which they have it in their power to communicate, either at a distance by means of reflexion, or by contact. But the result might be different in the event of contact with the nucleus if, as is probably the case, the matter of which the nucleus is composed should have become heated, in the neighbourhood of the sun, to so high a temperature as to cause its partial incandescence.

Arago's demonstration did not succeed in convincing every one, for after the apparition of Halley's famous comet, the mild temperatures of the months of October and November were ascribed by many persons to the passage of the comet. People wish,' he observes, 'to attribute the mild temperature enjoyed by the north of France during these eight weeks to the influence of the comet ! I could,' he continues, 'instance on the one hand Octobers and Novembers still milder than those of 1835, when no comets were visible, and on the other I could find instances of great cold being experienced during the same months, when brilliant comets were in sight; but to come more directly to the point, I will remark that at the end of 1835, when Paris was enjoying a very mild temperature, it was especially cold in the south, so that if the temperature were dependent upon the comet, its action would have to vary with the position of the place.'

And further, in order to judge the question fairly by this

method, that is to say, by the comparison of meteorological statistics, it is clear that we must not be content with observations relating only to one region of the earth. In order to form an impartial judgment, we must decide whether the presence or proximity of the comet corresponds to an increase of temperature over the whole of the terrestrial globe, or at least over all that portion of the globe which occupies the same relative position with regard to the comet.

The comet now in sight [July 1874] is observed by the public at the hottest time of the year, and it is probable that, without seeking further for a cause, many people attribute to the comet the high temperature from which they suffer. This present year may be in France and even throughout Europe a warm year. But is it so too for the same latitudes in America? Coggia's comet is the third of the year 1874; but we must not forget that in 1873 no less than seven comets passed their perihelia.

In conclusion, we may say that the influence of a comet upon the temperature and the seasons is generally imperceptible. It could only become sensible on the hypothesis of a collision, or a very near approach between the earth and a comet. Finally, up to the present time we have no authentic instance of such an influence. Mere opinions which are not justified by examination of the facts are but valueless hypotheses.

SECTION III.

PENETRATION OF COMETARY MATTER INTO THE TERRESTRIAL

ATMOSPHERE.

Is this penetration physically possible?-Cometary influences, according to Dr. Forster -Were the dry fogs of 1783, 1831, and 1834, due to the tails of comets ?— Volcanic phenomena and burning turf-beds; their probable coincidence with fogs-Probable hypothesis of Franklin-Dry fogs, atmospheric dust, and bolides.

WE perceive, then, that the influence of comets upon living beings by the action of heat is a hypothesis which, for the present, must be abandoned; in so far, at least, as the action of heat by radiation from a distance is concerned. We have throughout reserved the questions of a collision between the two bodies, and of the penetration of the earth to the heart of a mass in a state of incandescence.

Apart from the action of calorific radiation, what influence of any other kind could a comet exercise upon the meterological conditions of the earth? We know of absolutely none.

It remains, then, to consider the immediate physical or chemical influence of the cometary substance. It is not forbidden to our globe, as we have seen, to traverse the gigantic trains which form the tails of certain comets, nor to penetrate to a certain depth the vaporous atmosphere of some amongst them. Apart from these rencontres, we may suppose that cometary matter may be introduced into our atmosphere by the power of attraction. Pursuing its course in the same