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But while the theory of seed-bearing meteors can hardly be regarded as a complete solution of the perplexing problem of the origin of life, the facts to which the eminent Scottish professor referred while discussing it are of singular interest and importance. The whole history of recent scientific research into the subject of the relation between meteors and comets is full of instruction. The subject is discussed in two preceding papers. Moreover, in my · Essays on Astronomy' there are four papers containing a full account of the researches of Schiaparelli, Adams, Leverrier, and those other men of science who have placed meteoric astronomy in its present position. I propose here, therefore, to take for granted many of the conclusions dealt with in my former paper. This will enable me to discuss with greater freedom, as regards space, the views respecting comets, and more especially respecting cometic appendages, which seem to be suggested by observed phenomena, taken in connection with the association recently recognised between comets and meteors. The subject is as yet too new for the enunciation of definite theories, and still less can we safely dogmatise respecting it. But much has been established which will well bear careful investigation, and I believe that the conclusions which may be fairly deduced from observations already made are much more important than is commonly supposed.
The phenomena presented by comets have long perplexed astronomers. Setting aside the fact that the head of a comet strictly obeys the law of gravitation, there is scarcely one known fact respecting comets which astronomers have succeeded in interpreting to their satisfaction. The facts recently ascertained, striking and important though they undoubtedly are, yet not only fail to explain the phenomena of comets, but are absolutely more perplexing than any which had before come to light. The present position of cometic astronomy is, in fact, this :- Many facts are known, and many others may be inferred; but these facts have yet to be combined in such a way as to afford a consistent theory respecting comets.
It is now known that the comets which are so brilliant as to attract general notice are but a few among those which actually approach the earth. The telescope detects each year (with scarcely an exception) more than one comet. It is probable, indeed, that if systematic search were diligently made, many comets would be detected yearly. Already, however, nearly seven hundred comets have been discovered, of which by far the greater number have been the reward of modern telescopic research.
Of observed comets, only the more brilliant are adorned with tails of considerable length. But nearly all comets show, during their approach towards the sun, a certain lengthening of their figure, corresponding to the change which, in the case of larger comets, precedes the formation of a tail. So that a tail may be regarded as a normal, or at least a natural, appen
? A prize has been offered to the astronomer or telescopist who shall first succeed in discovering eight comets within the year.
dage of comets—though special conditions may be requisite for the evolution of the appendage. This will appear the more probable when the fact is noted that, in all cases where a tail is formed, this tail appears as an extension of the part of the head known as the coma or hair—the fainter light surrounding the nucleus of the comet—and no comet has ever appeared without showing a coma during one period or another of its existence. Commonly, the coma continues visible as long as the comet itself can be discerned, though there have been instances in which the comet seems to have been shorn of its hair; and, in one noteworthy instance, a comet of considerable splendour lost in a few days both its tail and hair.
Now when we consider the remarkable appearance which the tails of comets have presented, the great variety of their aspect, and the wonderful changes which have been noted in the appearance of one and the same comet, we begin to recognise the enormous difficulty of the problem which astronomers have to solve. It will be instructive to discuss some of these peculiarities at length, because they seem to oppose themselves in a very striking manner to theories which have been somewhat confidently urged of late.
In the earliest ages of the history of our subject, the fact was noted that the tails of comets commonly lie in the direction opposite to the place of the sun. Appian, indeed, was the first European astronomer who observed this peculiarity, but M. Biot has succeeded in proving that the discovery had been made long before by Chinese astronomers.
If the tail of a comet strictly obeyed this rule, if it were always directed in a perfectly straight line from the sun's place, the peculiarity might admit perhaps of a tolerably simple explanation. This, however, is not in general the case; in fact, I do not know of a single instance in which a comet's tail has extended exactly in the direction of a line from the sun throughout the tail's whole length. The tail of an approaching comet generally seems to bend towards the track along which the comet has recently passed, and the effect, when the tail is long, is to give the appendage a slight curvature. To cite only one instance out of many, it will be sufficient to refer to the splendid comet which appeared in 1858, and was known as Donati's. Soon after the first appearance of the tail a slight curvature could be recognised in the appendage; and this curvature became gradually more and more conspicuous, until, to use Sir John Herschel's words, the tail assumed at length that superb aigrette-like form, like a tall plume wafted by the breeze, which has never probably formed so conspicuous a feature in any previous comet.'
Here is a peculiarity which at once serves to dispose of the theory according to which the tail of a comet is to be compared to a beam of light such as a lantern throws amid darkness. The theory seems so naturally suggested by the general fact that a comet's tail tends from the sun, as to lead many to forget that the socalled beam of light thrown by a lantern is in reality due to the illumination of material particles; and that in the case of a comet we can neither explain why particles behind the comet (with regard to the sun) should be more brilliantly illuminated than others, nor how the particles come to be there at all. Despite these and other difficulties, the negative shadow' theory, as it has been called, has been again and again urged, though only to be again and again refuted.
Let it be noted, however, before other peculiarities are considered, that the curvature of comets' tails is no argument against the ingenious theory by which Professor Tyndall has endeavoured to explain their direction from the sun. According to this theory, the passage of light through and beyond the head of the comet is the real cause to which the appearance of the tail is to be ascribed. But a physical process is supposed to occur as the light traverses the region behind the comet; and the rate at which this process takes place need not necessarily correspond to the enormous velocity with which light travels. So that, instead of the whole tail being exactly in a straight line with the head and the sun, as it must be (appreciably) if the phenomenon were a mere luminous track, the end of the tail (the part formed earliest) would lie in the direction of a solar ray through the place occupied some time earlier by the head. This, in fact, corresponds somewhat closely with observed appearances ; and so far Professor Tyndall's theory receives undoubted support from recogmised facts.
Indeed, we seem almost driven to the conclusion that some such action as Tyndall has conceived takes