Perhaps they may in the end be so condensed as actually to become stars

The nebulous matter in a cubical space of 10′ will admit of a condensation of two trillion and 208 thousand billion times before it can go into the compass of a globe of the diameter of our sun

Planetary nebulæ have a rotatory motion on their axes

The original eccentricity of the nebulous matter of a nebula may be the physical cause of the rotatory motion of celestial bodies

The nebulous star in Orion is fictitious

99

Two out of three nebulous stars in Orion have lost their former nebulous appearance

100

Their fictitious appearance was owing to a dispersion of their light in passing through nebulous matter

100

The faintest appearance of the nebulosity in Orion is perhaps
not further from us than the stars of the 3d or 2nd magni-
tude; the brightest part is therefore probably not so far
In thirty seven years the nebulosity of this nebula has under-
gone great changes, and much greater since the time of
Huyghens

Nebulæ are not permanent celestial bodies

Additional proof of the opacity of the nebulous matter
Very distant nebulosities, which cannot be seen in a state of
diffusion, may become visible when condensed into separate
nebulæ

101

102

102

103

103

Conversion of planetary into bright stellar nebulæ, into stars with burs, or stars with haziness

104

Conversion of more distant ones into faint stellar nebulæ; into stars with burs, or with faint chevelure

106

When it is doubtful whether an object is a star or a nebula, it
may be verified by an increase of magnifying power
When the object is very like a star, it becomes difficult to as-
certain whether it is a star or a nebula

106

107

When we cannot ascertain whether the doubtful object is a star or a nebula, of which several instances are given, the similitude is as great as any we can expect; for were it greater, there could be no doubt

107

POSTSCRIPT.

It will be seen that in this paper I have only considered the nebulous part of the construction of the heavens, and have taken a star for the limit of my researches. The rich collection of clusters of stars contained in the 6th, 7th, and 8th classes of my Catalogues, and many of the Connoissance des Temps, have as yet been left unnoticed. Several other objects, in which stars and nebulosity are mixed, such as nebulous stars, nebulæ containing stars, or suspected clusters of stars which yet may be nebulæ, have not been introduced, as they appeared to belong to the sidereal part of the construction of the heavens, into a critical examination of which it was not my intention to enter in this paper.

DR. HERSCHEL supposes that the luminous fluid which so often

appears in different parts of the heavens, and throws streaks athwart them, is not light immediately issuing from stars too remote to be traced by the telescope, but existing independently of stars or planets, though perhaps originally thrown forth from them.

It is not matter of light alone, for it is sometimes capable of opacity, though usually luminous, and a source of light. When this subtle material exists in irregular loose masses he calls it a nebulo. sity, or luminous cloud. Its residence he supposes to be within the range of the remotest stars, though beyond those of the second and third magnitudes; yet similar diffusions, he conceives, may also exist at distances where they cannot be seen.

Agreeably to the laws of gravitation or the centripetal force, he supposes the different particles to have a tendency of approaching to each other, and that some circumstances, unknown to us, may occasion a preponderating influence, usually in the centre of a diffused mass, but sometimes in other quarters. Hence, a luminous nucleus will be produced by the concentration that must necessarily follow, which will progressively attract and determine the circumjacent luminous matter to itself, and diminish the extent of the general range; and in this case what was a nebulosity will become a nebula.

The nuclei may sometimes be double, or triple, or still more complicated in the same nebulosity, and whenever this occurs, the nebulosity will be broken into different nebulæ, or smaller luminous clouds; and if some of them be much minuter than others, they may ultimately attend upon them as satellites upon a planet. If the nebulæ have a hazy or superficial lustre, he calls them planetary; if they be uniformly luminous, he denominates them stellar.

Dr. Herschel conceives that stars, planets, and comets, may all occasionally originate from such a source; that a rotatory motion must ensue from the preponderancy of action of a greater mass of particles on one side of the nucleus than on other sides: and that, as many of these particles are probably elastic, an apparent haze or chevelure, coma or hair, must often appear to surround the nucleus. When the whole of the luminous circumferent matter is consolidated by gravitation into the nucleus, the planetary nebula becomes then a real star; some of which stars, before they become perfectly consolidated, have visibly faint chevelures, or else burs, or prickles. The nebulous matter contained in a cubical space seen under an angle of ten degrees, will admit of a condensation of two trillion and two hundred and eight thousand billion times, before it can be so consolidated as to constitute a globe of the diameter of

our sun.

The ground-work of these opinions, or the principles from which these results are drawn, will be found at length in the preceding chapter, and they are too curious and of too much consequence for us to diminish their force by an abridgment. Yet the younger and the less inquisitive among our readers, will feel obliged to us for presenting them, in the chapter before us, with a miniature view of the general information they contain.

Editor.

CHAP. IX.

ACCOUNT OF DR. HERSCHEL'S PAPER ON THE CHANGES THAT HAVE HAPPENED DURING THE LAST TWENTYFIVE YEARS, IN THE RELATIVE SITUATION OF DOUBLE STARS; WITH AN INVESTIGATION OF THE CAUSE TO WHICH THEY ARE OWING.

DR. Herschel devotes this paper principally to the consideration

of the second class of the systems into which he has divided the sidereal world. After cursorily remarking, with respect to the solar system, as a specimen of the first class, which, among the insulated stars, comprehends the Sun, that the affections of the newly discovered celestial bodies extend our knowledge of the construction of this insulated system, which is best known to us; he proceeds to support, by the evidence of observation, the opinion which he has before advanced, of the existence of binary sidereal combinations, revolving round the common centre of gravity; Dr. Herschel first cousiders the apparent effect of the motion of either of the three bodies concerned, the two stars, and the Sun with its attendant planets; and then states the arguments respecting the motions of a few only out of the fifty double stars, of which he has ascertained the revolutions. The first example is Castor, or alpha Geminorum: here Dr. Herschel stops to show how accurately the apparent diameter of a star, viewed with a constant magnifying power, may be assumed as a measure of small angular distances; he found that ten different mirrors, of seven feet focal length, exhibited no perceptible difference in this respect. In the case of Castor no change of the distance of the stars has been observed, but their angular situation appears to have varied somewhat more than 45° since it was observed by Dr. Bradley, in 1759'; and they have been found by Dr. Herschel, in intermediate positions at intermediate times. Dr. Herschel allows that it is barely possible that a separate proper motion, in each of the stars and in the Sun, may have caused such a change in the relative situation, but that the probability is very decidedly in favour of the existence

VOL. I.

of a revolution. Its period must be a little more than 342 years, and its plane nearly perpendicular to the direction of the Sun. The revolution of gamma Leonis is supposed to be in a plane considerably inclined to the line in which we view it, and to be performed in about 1200 years. Both these revolutions are retrograde; that of epsilon Bootis is direct, and is supposed to occupy 1681 years, the orbit being in an oblique position with respect to the Sun. In zeta Herculis, Dr. Herschel observed, in 1802, the appearance of an occultation of the small star by the larger one in 1782 he had seen them separate; the plane of the revolution must therefore pass nearly through the Sun; and this is all that can at present be determined respecting it. The stars of delta Serpentis appear to perform a retrograde revolution in about 375 years: their apparent distance is invariable, as well as that of the two stars which constitute ganıma Virginis, the last double star which Dr. Herschel mentions in this paper, and to which he attributes a periodical revolution of about 708 years.

[Young's Nat. Philosoph. Vol. II.-Journals of the Royal Institution, Vol. II.]

CHAP. X.

DR. HERSCHEL'S OBSERVATIONS OF THE COMET OF THE YEAR 1811, WITH REMARKS ON THE CONSTRUCTION OF ITS DIFFRENET PARTS.

THE Comet which has lately visited the solar system has moved in

an orbit very favourably situated for astronomical observations. I have availed myself of this circumstance, and have examined all the parts of it with a scrutinizing attention, by telescopes of every degree of requisite light, distinctness, and power.

The observations I have made have been so numerous, and so often repeated, that I shall only give a selection of such as were made under the most favourable circumstances, and which will serve to ascertain the most interesting particulars relating to the construction of the comet.