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EMERSION. Third Satellite, 26th day at 45 min. 2 sec. after 1 morning.

IMMERSION.
Fourth Satellite, 10th day at 29 min. 13 sec. after midnight.

FORM OF SATURN'S RING.
July 19th.-Semi-transverse axis 37".96

Semi-conjugate axis 2 .69
Uranus in nearly the same position as last month.

Sphere of the Fixed Stars. Positions of the principal constellations, at 11 in the evening of the 1st of the month.

On the meridian. Scorpio near the southern horizon: Above Scorpio are Serpens, Ophiuchus, Hercules, and the bead of Draco in the Zenith. North of the Zenith are Tarandus, Custos Messium, Camelopardalis, and Auriga; the latter in the horizon.

Perseus, N. N. E. Andromeda, N.E. Pegasus, E. by N. Cassiopeia, N.E. by N. Cygnus, East. Lyra, Delphinus, Equuleus and Aquarius, E.S.E. Aquila, Antinous, and the head of Capricornus, S. E. by E. Taurus Poniatowski, Scutum Sobieski and Sagittarius, S.S.E. Leo and Leo minor, W.N. W., above which is Ursa Major. Coma Berenices, West. Boötes and Virgo, W. S. W. Corona Borealis, Serpens and Libra, S.S. W.

Telescopic Objects. Libra. In this zodiacal constellation, a, and i are double stars,—the largest of is white, and the small star dusky red. & is a triple star,--with inferior telescopes this appears only a double star, the largest of which is very white; this is, however, itself a double

star. ɛ is a beautiful double-double star, or a double star, each star being itself a double star; the first set consists of stars that are very unequal, the largest is white, and the smallest reddish; the second set are white and equal.

North of & is a nebula.
k is supposed to be a variable star.

Scorpio. In this constellation B and v are double stars ; near v is a multiple star; near to the red star Antares is a mass of small stars ; o and 19 point to a nebula. In this constellation are some remarkable nebulæ, two of which are like comets, with brilliant centres. Scorpio is on the borders of the Via Lactea, which Herschel considered, as a very extensive branching congeries of many millions of stars; that probably it owes its origin to several remarkably large, as well as pretty closely scattered small stars, that may have drawn together the rest; also that there are many parts of the Via Lactea where the stars are drawing towards secondary centres, and may, in time, separate into different clusters. He also imagined, that there are some portions of the Via Lactea which have suffered greater ravages than others, and particularly that part of it in the body of Scorpio, where there is a large opening or hole, about four degrees broad, which is almost destitute of stars; and that the stars which once filled this vacancy is a rich cluster of small stars, just upon the western border of the opening:

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 depsity,—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 goooth 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 dersity 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.

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