stars B and y Lyræ, and may be seen with a telescope of moderate power. It is small, and particularly well defined, so as in fact to have much more the appearance of a flat oval solid ring than of a nebula. The axes of the ellipse are to each other in the proportion of about 4 to 5, and the opening occupies about half its diameter: its light is not quite uniform, but has something of a curdled appearance, particularly at the exterior edge; the central opening is not entirely dark, but is filled up with a faint hazy light, uniformly spread over it, like a fine gauze stretched over a hoop.

(623.) Planetary nebulæ are very extraordinary objects. They have, as their name imports, exactly the appearance of planets: round or slightly oval discs, in some instances quite sharply terminated, in others a little hazy at the borders, and of a light exactly equable or only a very little mottled, which, in some of them, approaches in vividness to that of actual planets. Whatever be their nature, they must be of enormous magnitude. One of them is to be found in the parallel of Aquarii, and about 5m preceding that star. Its apparent diameter is about 20". Another, in the constellation Andromeda, presents a visible disc of 12", perfectly defined and round. Granting these objects to be equally distant from us with the stars, their real dimensions must be such as would fill, on the lowest computation, the whole orbit of Uranus. It is no less evident that, if they be solid bodies of a solar nature, the intrinsic splendour of their surfaces must be almost infinitely inferior to that of the sun's. A circular portion of the sun's disc, subtending an angle of 20", would give a light equal to 100 full moons; while the objects in question are hardly, if at all, discernible with the naked eye. The uniformity of their discs, and their want of apparent central condensation, would certainly augur their light to be merely superficial, and in the nature of a hollow spherical shell; but whether filled with solid or gaseous matter, or altogether empty, it would be a waste of time to conjecture.

(624.) Among the nebula which possess an evident symmetry of form, and seem clearly entitled to be regarded as systems of a definite nature, however mysterious their structure and destination, the most remarkable are the 51st and 27th of Messier's catalogue. The former consists of a large and bright globular nebula surrounded by a double ring, at a considerable distance from the globe, or rather a single ring divided through about two fifths of its circumference into two laminæ, and having one portion, as it were, turned up out of the plane of the rest. The latter consists of two bright and highly condensed round or slightly oval nebulæ, united by a short neck of nearly the same density. A faint nebu lous atmosphere completes the figure, enveloping them both, and filling up the outline of a circumscribed ellipse, whose shorter axis is the axis of symmetry of the system about which it may be supposed to revolve, or the line passing through the centers of both the nebulous masses. These objects have never been properly described, the instruments with which they were originally discovered having been quite inadequate to showing the peculiarities above mentioned, which seem to place them in a class apart from all others. The one offers obvious analogies either with the structure of Saturn or with that of our own sidereal firmament and milky way. The other has little or no resemblance to any other known object.

(625.) The nebulæ furnish, in every point of view, an inexhaustible field of speculation and conjecture. That by far the larger share of them consist of stars there can be little doubt; and in the interminable range of system upon system, and firmament upon firmament, which we thus catch a glimpse of, the imagination is bewildered and lost. On the other hand, if it be true, as, to say the least, it seems extremely probable, that a phosphorescent or self-luminous matter also exists, disseminated through extensive regions of space, in the manner of a cloud or fog-now assuming capricious shapes, like actual clouds drifted by the wind, and now con

centrating itself like a cometic atmosphere around particular stars; what, we naturally ask, is the nature and destination of this nebulous matter? Is it absorbed by the stars in whose neighbourhood it is found, to furnish, by its condensation, their supply of light and heat? or is it progressively concentrating itself by the effect of its own gravity into masses, and so laying the foundation of new sidereal systems or of insulated stars? It is easier to propound such questions than to offer any probable reply to them. Meanwhile, appeal to fact, by the method of constant and diligent observation, is open to us; and, as the double stars have yielded to this style of questioning, and disclosed a series of relations of the most intelligible and interesting description, we may reasonably hope that the assiduous study of the nebulæ will, ere long, lead to some clearer understanding of their intimate nature.

(626.) We shall conclude this chapter by the mention of a phænomenon, which seems to indicate the existence of some slight degree of nebulosity about the sun itself, and even to place it in the list of nebulous stars. It is called the zodiacal light, and may be seen any very clear evening soon after sunset, about the months of April and May, or at the opposite season before sunrise, as a cone or lenticular-shaped light, extending from the horizon obliquely upwards, and following, generally, the course of the ecliptic, or rather that of the sun's equator. The apparent angular distance of its vertex from the sun varies, according to circumstances, from 40° to 90°, and the breadth of its base perpendicular to its axis from 8° to 30°. It is extremely faint and ill defined, at least in this climate, though better seen in tropical regions, but cannot be mistaken for any atmospheric meteor or aurora borealis. It is manifestly in the nature of a thin lenticularly-formed atmosphere, surrounding the sun, and extending at least beyond the orbit of Mercury and even of Venus, and may be conjectured to be no other than the denser part of that medium, which, as we have reason to believe,

resists the motion of comets; loaded, perhaps, with the actual materials of the tails of millions of those bodies, of which they have been stripped in their successive perihelion passages (art. 487.), and which may be slowly subsiding into the sun.

CHAP. XIII.

OF THE CALENDAR.

(627.) TIME, like distance, may be measured by comparison with standards of any length, and all that is requisite for ascertaining correctly the length of any interval, is to be able to apply the standard to the interval throughout its whole extent, without overlapping on the one hand, or leaving unmeasured vacancies on the other"; to determine, without the possible error of a unit, the number of integer standards which the interval admits of being interposed between its beginning and end; and to estimate precisely the fraction, over and above an integer, which remains when all the possible integers are subtracted.

(628.) But though all standard units of time are equally possible, theoretically speaking, all are not, practically, equally convenient. The tropical year and the solar day are natural units, which the wants of man and the business of society force upon us, and compel us to adopt as our greater and lesser standards for the measurement of time, for all the purposes of civil life; and that, in spite of inconveniencies which, did any choice exist, would speedily lead to the abandonment of one or other. The principal of these are their incommensurability, and the want of perfect uniformity in one at least of them.

(629.) The mean, lengths of the sidereal day and

year, when estimated on an average sufficiently large to compensate the fluctuations arising from nutation in the one, and from inequalities of configuration in the other, are the two most invariable quantities which nature presents us with; the former, by reason of the uniform diurnal rotation of the earth-the latter, on account of the invariability of the axes of the planetary orbits. Hence it follows that the mean solar day is also invariable. It is otherwise with the tropical year. The motion of the equinoctial points varies not only from the retrogradation of the equator on the ecliptic, but also partly from that of the ecliptic on the orbits of all the other planets. It is therefore variable, and this produces a variation in the tropical year, which is dependent on the place of the equinox (arts. 517. 328.) The tropical year is actually above 4.21s shorter than it was in the time of Hipparchus. This absence of the most essential requisite for a standard, viz. invariability, renders it necessary, since we cannot help employing the tropical year in our reckoning of time, to adopt an arbitrary or artificial value for it, so near the truth, as not to admit of the accumulation of its error for several centuries producing any practical mischief, and thus satisfying the ordinary wants of civil life; while, for scientific purposes, the tropical year, so adopted, is considered only as the representative of a certain number of integer days and a fraction—the day being, in effect, the only standard employed. The case is nearly analogous to the reckoning of value by guineas and shillings, an artificial relation of the two coins being fixed by law, near to, but scarcely ever exactly coincident with, the natural one, determined by the relative market price of gold and silver, of which either the one or the other,whichever is really the most invariable, or the most in use with other nations,—may be assumed as the true theoretical standard of value.

(630.) The other inconvenience of the standards in question is their incommensurability. In our measure, of space, all our subdivisions are into aliquot parts: a