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require a large telescope and a practised eye to see it; and even then it appeared only as a dull nebulous spot about three minutes in diameter, a little condensed towards the centre, and shaded off indefinitely at the edges. It was best seen with the eye-piece powers of 50 and 22 times, and tolerable places were obtained by an eye-piece armed with a broad bar instead of wires, so as to render the gazer independent of an illuminated field of view. (See Plate XIII.)
At the very name of comet what a host of associations and ideas arise; what a mass of human errors and perversities does the history of cometology exhibit; what fear and dread have those wanderers so needlessly inspired ! Never did popular prejudice adopt more unlikely engines for producing the threatened destruction of the world: without light or heat of their own, without any solidity, and without appreciable weight, how utterly powerless they must be for good or for evil !
We now understand them to be as orderly members of the solar system as the planets: like them, and yet different from them, bearing the same relation to them that birds do to animals, both denizens of our earth, but the former able from their lighter nature to rise and perform flights in a medium into which the latter may never elevate themselves. So the comets-circulating like the planets round the self-same luminary, yet not affected by their grosser weight, take flights above and below the ecliptic at all angles and in all directions; and with velocities and ex-centricities that seem for ever to be denied to the others.
Some have attempted to make out a connexion between planets and comets: and there were even hopes, before the discovery of Uranus and Neptune on the one hand, and of small comets of short period on the other, that on the outer confines of the system the ex-centricities of planets there to be found might approximate and mingle with those of comets. Recent discoveries have, however, shewn the outermost planets to have the least ex-centricity, and the outermost comets the most ; each of these respective species thus shewing no tendency whatever to any amalgamation. But it is far more in the physical features of the comets, than the character of their orbits, that
they differ from planéts: and the latter's motions are understood, while the former's qualities are still as imperfectly known as ever they were in those days when they spread alarm and dismay amongst nations.
That so little advance has been made in this department seems to be due chiefly, among those causes which we can modify, to too much having been attempted; to inquiries having been made into matters not strictly within the reach of science. Thus, it was not enough to make statistical registers of the various genera of comets, and to investigate the routine of changes which each undergoes in the various points of its orbit, but men must needs account for how a comet was created, and what purpose it was subserving. In the dark ages these purposes were to foreshadow the deaths of kings, to indicate revolutions and wars, and—when these ideas were dissipated before an advancing philosophy—frosts, storms, heats, droughts, &c. took their place. These, again, have vanished before the onward march of science; but, such is the perverse vitality of error, that men still think themselves bound to supply some cause and some purpose for the existence of comets. Thus, in the excellent work recently published under the title of Outlines of Astronomy, § 554, the illustrious author says,—“Even now that we have ceased to regard their movements as irregular, or as governed by other laws than those which retain the planets in their orbits, their intimate nature, and the offices they perform in the economy of our system, are as much unknown as ever. No distinct and satisfactory account has yet been rendered of those immensely voluminous appendages which they bear about with them, and which are known by the name of their tails (though improperly, since they often precede them in their motions), any more than of several other singularities which they present.”
Here now there seem to be far more stringent requirements than are exacted in the case of the planets; for who has ever thought it a drawback on our knowledge of planetary astronomy, that we did not know the office which these bodies perform in the economy of the system? Enough for us, then, that we can determine with greater exactness every year, their distance, size, weight, the form of their surface, with the motions and characteristics of their atmo
spheres; and the sooner we apply ourselves to steadily accumulating similar data respecting comets the better; for certain glimpses of the wished-for desiderata may then appear of themselves. Royal roads and short cuts have, however, seductive charms for the despisers of induction : and I will here give some passages of a letter lately written (August, 1850), by an Assistant-Astronomer in a standard observatory, as a sample of side-wind philosophy :
Je m'occupe avec les sciences physiques et j'ai fait quelques découvertes très intéressantes, dont vous verrez bientôt l'une qui exprime la cause du magnétisme terrestre.—J'ai trouvé aussi un système pour supprimer les incendies. Ce système a été déja approuvé par le Comité “ select” de Woolwich, et à present je m'occupe à le mettre en exécution.—J'ai trouvé que la terre et les planètes subirent plusieurs périodes géologiques, et pendant chaque période des ages sont parcourus ; qui se distinguent par leur températures. Le nombre des périodes parcourues par chaque planète est different, quoique elles ont été produites par le soleil à la même époque. Maintenant, les planètes parcourent des ages différentes. Saturne parcoure son age glaciale, Jupiter son age temperé, et Mars son age torride, la Terre se trouve dans son age temperé. Ces ages dépendent de l'épaisseur de l'atmosphère, qui se forme par la décomposition de l'eau. L'azote n'est pas un corps simple, mais un composé de 2 hydrogène et 1} oxygène. 4 atomes d'eau = 40 + 4 H = 0 + 2 (1:0 + 2 H) = 0 + 2 A 2 = air. Ainsi les mêmes elements produisent l'eau et l'air.
Les coinètes sont des atmosphères détachées en deux moitiés par les poles des planètes à la fin de chaque période géologique, ou au moment de cette catastrophe par la séparation de l'atmosphère, les corps organisés cessent de vivre, la température baisse, et les eaux accumulées dans la zone torride
par la pression de l'atmosphère dans les latitudes supérieures, ou son épaisseur était la plus grande, se déchainerent, et elles formerent des torrents dirigés vers les pôles, qui apportèrent les débris des terreins pour ensevelir les cadavres dont la surface de la terre était jonchée. On a été étonné ici dans les musées quand on a entendu qu'il n'existe pas des fossiles d'animaux terrestres dans la zone torride.—Les élémens des comètes sont les mêmes que ceux de l'atmosphère, 1° de l'eau, 2° des vapeurs, et 3° de l'air. Ainsi les comètes ne sont que des vents de l'espace.
Here is a precious theory, built up without a single fact, and at variance with all experience! First of all azote, suspected to be a compound gas, but never yet proved to be so, has its composition exactly laid down from fancy, so as to agree with a supposed easy plan for nature to manufacture the gas on a large scale. Then we come by a jump to the comets, which are portions of the atmosphere detached from each pole of the earth, at stated periods. As to how this detachment is brought about, why it should be, and when, not a glimpse is given; nor of how the detached air is at once to take upon itself the peculiar motions and characteristics of a comet, so excessively different to all that the body of air had been accustomed to, when still accompanying the earth. All these difficulties are, however, overlooked, and the notable conclusion is at length come to, that the elements of comets are the same as those of the atmosphere, and that thus comets are but the winds of
If, instead of such constituent principles as these, some additional good honest elements in the shape of perihelion distance, excentricity of orbit, &c. of comets had been given, some real approach to a knowledge of the physical characteristics of these bodies might have been obtained.
Investigations into the orbits of comets can fortunately be pursued without any knowledge of their form and size, by mere reference to the point called the nucleus; all the sensible weight of the body is so completely centered here, that, however much further the body may extend on one side than on the other, it makes no difference, no sensible difference at least, in the position of the centre of gravity and of motion. With the planets, we may either observe the centre, or the two limbs, and the mean of these will be seen to give the centre, but it is far otherwise with the comets : at the tail side the body may extend sixty to one hundred degrees from the nucleus, at the other side only one-fourth of a degree; but yet the nucleus moves as if wholly independent of all that excentric body. This body, both the tail part and the denser parts about the nucleus, called variously the coma or the head, and often mistaken for the nucleus itself, are constantly altering in size and form; the nucleus is the only portion which remains constant; it is infinitely small in size, is seen only as a minute stellar point, and to this alone the attribute of solid matter can attach. To this nucleus proper (not the head, as noticed by old observers) no sensible magnitude has ever been attached, and every succeeding improvement in telescopes further limits the size which it can possibly be of. The smaller comets do not show any apparent nucleus; the existence of one, however, though too small to be seen even in our best telescopes, may always