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COMETARY ASTRONOMY.

Density of Comets. There are the strongest reasons for believing that the density of that part which is termed the nucleus, or solid body of a comet, is very inconsiderable, and contains but little matter in proportion to its bulk. In this respect they differ most widely from the planets, some of which are bodies of great density, the earth is four and a half times more dense than the sun, and Mercury twice as dense as the earth; the higher planets are also dense bodies, though not so much so as the earth and the planets within its orbit. This is evident from their mutual influence on each other, Jupiter and Saturn, when in conjunction, alternately retard and accelerate each others motions. When it is considered what a vast number of comets are constantly traversing the system, it will be evident that they are bodies of very little density, or they would, long before this, have had some influence on the planetary system, by deranging their motions when passing in their neighbourhood; this, however, has not been the case, the comet of 1770 passed through the system of Jupiter without causing the least perceptible derangement of the planes of motion, or periods of revolution of the satellites; the same comet passed at no very great distance from our earth; in fact, it approached nearer than any other that has been observed: this close approximation furnished data for computing the limit of its mass; if the quantity of matter in the comet had been equal to that in the earth, it would have shortened the length of our year by one-ninth of a day: now this

effect was not produced, nor, indeed, was any perceptible perturbation detected; the solar tables, for that year, prove that no alteration could have taken place exceeding 2", from hence it followed that the mass of this comet was less than th part of the mass of the earth.

It will readily be admitted, that if the solid part, or nucleus of a comet, is of little density, that of the tail must be much less so, through which the smallest stars are visible without much diminution of their light; this fact alone sufficiently attests its highly attenuated nature. Newton calculated that if all the matter, constituting the largest tail of any comet, were to be compressed to the same density with our atmosphere, it would occupy no more than the space of a cubic inch! This statement may, to some, appear incredible; the truth of it, however, stands upon as firm a basis as any of the most universally received in the science of astronomy. The statements of astronomical science are frequently considered, by the uninformed, as visionary; to specify one, the distances of the planets from the sun, and from one another: many are unable to conceive of how the distance of an object can be determined, to which the standard of measurement cannot be extended. But astronomers have this measuring rod,-the earth's diameter, which they extend to the sun and planets, and with it measure the millions of miles that separate our globe from the neighbouring bodies; when this fails, they take a longer line of comparison,-the diameter of the earth's orbit, and attempt to measure the wondrous space that separates our sun from the nearest fixed star:

not till this measure fails does the astronomer yield his mind to the influence of conjecture. In the calculation just referred to, relative to the matter of a comet's tail, it can be demonstrated, that if a cubic inch of air be elevated to the height of the semi-diameter of the earth, it would diffuse itself over the planetary regions, and extend beyond the orb of Saturn.

Who marshals this bright host? enrolls their names,
Appoints their post, their marches, and returns,
Punctual at stated periods? who disbands
These veteran troops, their final duty done,
If e'er disbanded?-He, whose potent word,
Like the loud trumpet, levied first their powers
In Night's inglorious empire, where they slept
In beds of darkness; armed them with fierce flames;
Arranged, and disciplined, and clothed in gold,
And called them out of Chaos.

A TABLE

OF THE

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

Day.

A TABLE

OF THE

TRANSITS OF THE MOON AND PLANETS.

Moon. Mercury. Venus. Mars. Jupiter. Saturn. Uranus.

AUGUST.

The soul that sees Him, or receives sublimed
New faculties, or learns, at least, t'employ,
More worthily, the powers she owned before,
Discerns in all things what, with stupid gaze
Of ignorance, till then she overlooked,-
A ray of heavenly light, gilding all forms
Terrestrial in the vast and the minute;
The unambiguous footsteps of the God,
Who gives its lustre to an insect's wing,
And wheels his throne upon the rolling worlds.
Much conversant with Heaven, she often holds
With those fair ministers of light to man,
That fill the skies nightly with silent pomp,
Sweet conference.

SOLAR PHENOMENA.

Cowper.

The sun enters Virgo at 54 min. after 4 of the morning of the 23rd of this month.

The sun and all the planets move round the common centre of gravity of the system: which centre is nearly in the centre of the sun. This motion changes into epicycloids, the ellipses of the planets, and comets which revolve round the sun.

A body which weighs one pound at the equator of