EDITOR'S PREFACE.

I HAVE great pleasure in introducing to English readers M. GUILLEMIN'S valuable and interesting work on comets. When rapid progress has been made in any branch of science, it is generally very difficult for anyone, who has not been actually concerned in the investigations in question, to obtain accurate information of the state of our knowledge; and for this reason a book, such as the present, which gives an account of the new results that we owe to very recent researches, really confers a benefit upon many persons who, though taking a strong interest in the subject, have necessarily been quite unable to follow its development in the periodical publications of English and foreign scientific societies. There is no work that at all occupies the ground covered by that of M. GUILLEMIN; and as the subject is one which, always of high interest, has in the last few years acquired great importance in consequence of Schiaparelli's discovery of a connexion between comets and shooting-stars, I was anxious that it should appear in our language.

Whenever I have thought that additional explanation was desirable, or that the researches of the two years that have elapsed since the publication of the original work threw further

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light upon the subject, I have added a note of my own, such notes being always enclosed in square parentheses [ ]; and although I must not be understood to endorse M. GUILLEMIN'S conclusions in every case where I have not added a note, still I may say that there are very few of his views from which I should feel at all inclined to dissent. Of course I have corrected in the text all errors I have met with, which were evidently purely accidental, and such as always will occur in the first edition of any work. In two cases I have ventured to make more lengthy additions of my own-these relate to Coggia's comet, which had only just left us when M. GUILLEMIN's work was published, and the connexion of comets and shooting-stars. Some remarks will also be found in the note which follows the catalogue of comets at the end of the book.

In conclusion, I must express my thanks to Dr. Warren De LA RUE, F.R.S., for having very kindly placed at my disposal copies of his beautiful drawings of the great comet of 1861, which add greatly to the value of the work.

BLACKHEATH, S.E.: Nov. 15, 1876.

JAMES GLAISHER.

PREFACE.

our own.

THE UNIVERSE is formed of an infinity of worlds similar to The thousands of stars which meet our gaze in the azure vault of the heavens when we contemplate it with the naked eye, and which may be reckoned by hundreds of millions when we explore its depths by the aid of the telescope, are suns. These foci of light, these sources of heat, and incontestably of life, are not isolated; they are distributed into groups or clusters; sometimes by twos or threes, sometimes by hundreds, sometimes by myriads; the clouds of vaporous light called nebulæ are for the most part thus constituted.

Isolated or in groups, the stars seem to us immovable, so prodigious is the distance by which they are separated from the earth and from our sun. They move nevertheless; and amongst those whose velocities have as yet been measured may be reckoned some which are moving ten times and even fifty times quicker than a cannon-ball when it leaves the cannon. Movement is, therefore, the most universal law of the stars.

In like manner our sun moves through space and compels the earth to follow. He bears along with him, in this voyage through the boundless ether, the globes which form his cortége and gravitate about his enormous mass. During the thousands years that man has been a witness-an unconscious witness,

of

it is true of this circumnavigation of the universe, he has seen no change in the aspect of the surrounding worlds; the sidereal shores of the ocean in which this fleet of more than a hundred celestial bodies pursues its way preserves to all appearance its unchanging front. The immensity of the sidereal distances, it is well known, is the sole cause of this apparent immobility.

The solar world is, therefore, separated from all other worlds by unfathomable abysses; the sun is as it were isolated, lost in a corner of space, far from the millions of stars with which nevertheless it forms a system. Member of an immense association, integral molecule of the most vast, to all appearance, of the nebulæ, the Milky Way, the central star of our group seems to have no other mode of communicating with its coassociates than by the reciprocal exchange of undulations, that is to say, by the exchange of light and heat. Like disciplined and devoted soldiers, the earth and the planets march in company with the sun, effecting with marvellous regularity their nearly circular revolutions around their common focus, and never deviating from the limits imposed upon them by the law of gravitation.

They remain, therefore, isolated like the sun, separated from other sidereal systems by distances so enormous that the mind is powerless to conceive of them.

A relation, however, exists between our system and these systems, as we have just mentioned: the sun is a star of the Milky Way. But, we may ask, has the solar world no closer and more direct connexion with the rest of the visible universe?

The movement of translation by which it is animated proves at least that in some quarter of the heavens there is either an unknown celestial body, or a system of celestial bodies, around which gravitation causes the group to describe an orbit of undetermined period. And this movement of the

whole results from the concurrent action of all the stars in the universe. The force of gravitation is, therefore, a common bond of union between our world and all others.

Is it, therefore, steadily advancing to some celestial archipelago which it will finally attain in a few millions of years? Are, then, future generations destined to see other suns, from other points of view? These are questions whose solution may be considered inaccessible to us.

But, amongst the stars of which the solar system is composed, are there not some less immutably attached than the planets and the earth to the focus of their movement? Are there not some which depart to a greater distance from their focus, and which, like messengers detached from the group, carry to neighbouring world news of our own?

Such a hypothesis is not without foundation.

Astronomers, in fact, have for the last two centuries. studied the movement of certain celestial bodies, which come to us and gravitate about the sun, but which, after having, so to speak, saluted on their way the ruler of the planets, return and plunge again to immeasurable distances in the depths of ether. A small number of these stars, retained by the solar power, diverted from their path by the influence of some of the larger planets, have remained tributaries of the group of which they now form an integral part.

These singular stars, long disowned, are COMETS.

I have said long disowned. Comets, indeed, have only been considered during the last two centuries as properly belonging to the family of the stars: before Newton's time they were regarded even by astronomers as transient meteors, whose appearance, disappearance, and movements were subject to no law. For the ancients, and the world in general during the Middle Ages, and even during the Renaissance, they were objects of fear, miraculous apparitions, signs the precursors