صور الصفحة
PDF
النشر الإلكتروني

exactly the direction in which rays of light proceed from the sun.

But the path of a comet is curvilinear, and this figure of its course will account for a phenomenon observed in its tail, that it is not exactly opposite to the sun, and is also curved at the extremity; the nebulous projection is acted upon by two forces,-the impulsion of the solar rays, and the centrifugal principle by which a comet describes a curvilineal path; it must also be observed, that the nucleus, or more dense part, is especially influenced by the sun's attraction, and that the motion of the tail is governed by the attraction of the nucleus. The curve-like form in the tail is most conspicuous when the comet is nearest the sun, which then moves quickest, consequently, the tail is behind that place to which a line would be directed if drawn from the sun to the comet.

In the tails of some comets there have been observed a dark line, proceeding to some distance from the nucleus; in others, this part has been distinguished for its superior brilliancy; such were the appearances observed in the comets of 1577 and 1618, while the comet of 1744 was observed to assume both of these appearances at different times. Some comets have exhibited corruscations in their tails, which singular appearance was observed in the comets of 1680; that of 1744 sparkled forth, or vibrated luminous particles. These remarkable phenomena might have been owing to physical changes in these bodies, though, doubtless, much is to be attributed to the state of the terrestrial atmosphere.

The tails of comets vary very considerably in their forms, some straight, others curved, some whose sides

are parallel, some expanding like a fan, others bifurcated. In the comet of 1744 the tail was divided into two branches, one part was 7° or 8° long, and the other 24°. In the comet of 1769 a stream appeared on each side of the tail, proceeding from the nucleus; the sides of these streams, or lesser tails, were parallel to each other, and between them could be distinguished a darkish space, through which telescopic stars could be distinctly perceived. An appearance, of a similar nature, was observed in the comet of 1811. In the second comet of 1825 the tail was bifid, being divided into two great branches, going off immediately from the nucleus at an angle of 45° with each other. (See plate, Second comet of 1825.)

Though the fact of the rotation of comets is uncertain, that of the rotation of the tail is sufficiently proved, at least, in one instance, that of the comet of 1825,one of the streams of the tail returned to the same form and position within a certain time, from whence it was concluded, that the period of rotation was twenty hours and a half; it may be suggested that this rotation of the tail might also have been accompanied by a rotation of the comet, of which the axis of the tail might have been the axis of revolution.

The tails of comets are hollow, which idea is suggested by the nature of the solar action, which can only affect those parts more immediately exposed to its influence, namely, that hemisphere of the comet which is on the side next the sun, the plane of which may be considered as the frustrum of a hollow cone; the appearance of the tail warrants this conclusion, the upper and under sides are better defined, and more brilliant than the middle,

.

as on the supposition of their being hollow, more rays would be seen at the edges where the eye passes through them obliquely, than in the middle, which is the plane of direct vision. In the comet of 1769, just referred to, there can be no doubt but that there were two of these hollow cones, the one within the other; the outer one very thin, and only seen at the edges, where it appeared as two streams of light.

Often-so much I loved to trace
The secrets of this starry race-
Have I at morn and evening run
Along the lines of radiance spun,
Like webs, between them and the Sun,
Untwisting all the tangled ties
Of light into their different dyes.-
Nay, oft, so passionate my chase
Of these resplendent heirs of space,
Oft did I follow-lest a ray

Should scape me in the farthest night-
Some pilgrim Comet, on his way

To visit distant climes of light;

And well remember how I sung
Exulting out, when on my sight

New worlds of stars, all fresh and young,
As if, just born of darkness, sprung.

"Loves of the Angels."

A TABLE

OF THE

SUN'S RISING AND SETTING, RIGHT ASCENSION, DECLINATION, AND EQUATION OF TIME.

[blocks in formation]

JUNE.

SOLAR PHENOMENA.

The sun

Rose, visiting the earth with light, and heat,

And joy; and seemed as full of youth, and strong
To mount the steep of heaven, as when the stars
Of morning sung to his first dawn, and night
Fled from his face: the spacious sky received
Him blushing as a bride, when on her looked
The bridegroom: and spread out beneath his eye,
Earth smiled.

Pollok's" Course of Time.”

The sun attains his greatest northern declination, (23° 27' 35",) and also enters Cancer at 29 minutes after 11 of the morning of the 21st. At this time is the midday of the north, and the midnight of the south pole, and during several rotations of the earth on its axis the whole of the arctic regions enjoys the presence of the sun, and the antarctic experiences the deepest gloom of the long and dreary night of winter On the verge of the arctic circle the sun, at midnight, conceals for a short space of time one half of his disc, and then reascends to pour forth a flood of splendor, over every

« السابقةمتابعة »