of the zones may intersect that part of the earth's orbit through which it passes in August and November. When the earth encounters a thin portion of the zone the showers are scanty and if the intervals in the zone are wide, only scattered meteors will be visible, as on ordinary nights.

This zone of planetoids may be as old as the larger planets, and may rank as an important portion of the system. It may consist of those portions of matter which were not sufficiently near one another to be attracted into

one mass.

It is possible that this revolving zone may be composed not of solid bodies, but of nebulous matter like the tails of comets. We can more easily understand the disappearance of nebulous than of solid matter in our atmosphere, and a very large proportion of meteors never touch the earth.

It has also been suggested that meteors may have their origin in the zodiacal light, a phenomenon hereafter to be described. Since the plane of this nebulous substance is not parallel to the ecliptic, the earth might pass through it at one season, and be remote from it another. But this does not account for the appearance of shooting stars at all seasons of the year.

The interruption of these phenomena may be caused by a motion of the nodes of the stream of aerolites, so that what has at former periods been so striking, and what has been repeated in our own times, will again recur after an interval.

§ 151. The zodiacal light and meteors, although very unlike one another in appearance, may perhaps arise from similar causes. The zodiacal light is a pale cone of light projected from the sun after the evening and before the morning twilight. It is almost constantly visible in the torrid zone, but in northern temperate regions, is only distinctly visible in the beginning of spring, after the evening twilight, and at the end of autumn before the commencement of the morning twilight. Its light resembles that of a comet. The faintest stars may be seen through it. It is less bright than the milky way, with ill defined edges, scarcely to be distinguished from twilight. Hum

boldt describes it in 10° latitude as appearing very regularly about an hour after sunset. Before this, even if the night was perfectly dark, no trace of it could be seen. Then it suddenly became visible, extending in great brightness and beauty between Aldebaran and the Pleiades and attaining an altitude of 39°. Long narrow clouds appeared low down on the horizon, as if in front of a golden curtain, while bright tints played on the upper clouds. The light diffused in that part of the heavens appeared almost to equal that of the moon in her first quarter. When its brightness was greatest a mild reflected glow was visible in the east. Towards ten o'clock it became very faint, and at midnight only a trace of it remained.

§ 152. Its figure agrees with that of a spheroid seen in profile. It has the sun for its base, and its axis lies nearly in the direction of the zodiac whence it takes its name. It also lies very nearly in the plane of the sun's equator.

As the sun's equator is differently inclined to the hori zon, on account of the different positions of the sun in the ecliptic, the zodiacal light inclines with it, and is in a great measure concealed beneath the horizon; or at least its lustre is diminished by vapors. In the vernal equinox the arc of the ecliptic which the sun is about to enter is more elevated above the horizon of a place in north latitude than the equator is. The zodiacal light is then elevated above the equator by all the obliquity of the ecliptic; no other position is so favorable for our climate. In the summer solstice the arc of the ecliptic, and consequently the luminous cone, is parallel to the equator, and therefore much more inclined to the horizon of places in north latitude than in the spring.

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 varies from 8° to 30°. It must involve Mercury and sometimes Venus and the Earth, and if it were not extremely rare, would produce some disturbance in their motion, but in fact it does not appear to impede the progress even of the tails of

comets.

As to its probable composition we must choose between the supposition of its being purely nebulous, or loaded with the tails of millions of comets; or of its consisting of a stream of countless planetoids or meteors, too small to be seen separately, but able from their numbers to give out a faint light. This latter hypothesis has this advantage, that it resembles the cause assigned for periodic shooting

stars.

CHAPTER VIII.

COMETS.

The number of recorded Comets. Variety in their motions and appearance. Their immense size. Description of a Comet. The tails of Comets. Bessel's Theory of their formation. Halley's Comet. Biela's and Encke's. Their resistance by the ether. The mutual influence of Comets and Planets. Mass of the Comet of 1770. The probable effect of a collision with a Comet.

§ 153. Comets form a class of bodies entirely distinct from the fixed stars and from the planets, whether we regard the character of their movements or their physical constitution. They receive their name from their hairy appearance, caused by the coma or atmosphere which surrounds them. Of the number of comets it is impossible to speak with certainty. Many comets on their nearest approach to the sun are too distant to be seen from the earth; many may not have reached their perihelion within the recorded experience of man; many may be invisible from their diminutive size; many can be seen only from the south side of the equator, where there are but few means of observation; many, though on the north side of the equator, rise above the horizon only during the day; many pass unnoticed, owing to cloudy weather. Several have however been seen so bright as to be visible in the day time, even at noon and in bright sunshine; and there

is one instance on record of a very large one observed near the sun, when eclipsed, in the year 60 before Christ. The number of comets which enter our system must amount to many thousands; more than six hundred have been actually observed, and the orbits of between one and two hundred have been calculated.

They come from every region of the heavens, and move in every variety of plane. Some move in the same direction with the planets; others in the opposite direction. Some of them remain in sight for a few days only, others for many months; some move with extreme slowness, others with extraordinary velocity. Not unfrequently the two extremes of apparent speed are exhibited by the same body, in different parts of its course.

§ 154. Not only does a comet vary in its physical appearance and its speed in different parts of one course, but it sometimes presents on its return, an appearance so different as to be scarcely recognizable. Its size and splendor are sometimes so much diminished that it is diffi cult to identify it.

We must not however suppose that all the apparent changes in the tails of comets are real. Many of them are owing to the state of our atmosphere, as is proved by the same comet's appearing of different brilliancy and extent, in different parts of the globe. Our atmosphere is a coarse medium, through which to view objects so delicate.

Another circumstance which makes it difficult to identify a comet is, that their orbits are liable to change after they enter the solar system, owing to the attraction of the planets. The orbit of a comet is, however, more to be relied on, as a test of its identity, than its physical appearance, if the changes in its orbit can be accounted for by the influence of any member of the planetary system.

§ 155. We will now consider, in detail, the physical constitution of comets, their motions and the influence they may impart to, or receive from, the members of the solar system.

Comets are the most voluminous and at the same time the lightest bodies of our system.

The tails of some of the largest have extended over a

distance of from thirty to forty million leagues, a length much exceeding the interval between the sun and the earth. At the same time their weight is so slight that not one has disturbed a planet or a satellite, in the slightest perceptible degree. It follows that matter so exceedingly diffused must be transparent. Stars of the sixteenth and seventeenth magnitude, which the slightest fog would conceal, may be seen through their substance; and yet their light passes through thousands of miles of the body of the

comet.

It has been doubted whether comets shine with a light of their own or by reflected light like planets. As they present no phases like the moon, it has been supposed that they originate light. They may however reflect it throughout their whole substance like the light clouds of our atmosphere, which often appear soaked in light. The fact that part of their light is polarized makes it certain that at least part of their light is reflected.

§ 156. Comets consist of a large ill defined mass of cloudy luminous matter, usually increasing in brilliancy toward the centre. This central portion is called the nucleus of the body. The nucleus and coma belong to comets in all parts of their orbit. The tail and head are developed as they approach the sun.

The Nucleus of the comet is generally to be distinguished by its forming a comparatively bright point in the centre of the head. In most instances it has the appearance of a solid body, and frequently subtends an angle capable of telescopic measurement. It is usually enveloped in a dense nebulous stratum, called the coma. This stratum so frequently renders the edge of the nucleus indistinct, that it is extremely difficult to ascertain its diameter with any precision.

But though comets, in general, possess this nucleus or body, there are many of them in which it seems to be entirely wanting, and which present only a nebulous mass, having a gradual condensation towards the centre. Apparently, there is a regular gradation of Comets, from such as are composed merely of a gaseous or vapoury medium, to those which, by the mutual attraction and consolidation