SECTION XI.

THE ASTEROIDS.

123. BETWEEN the orbits of Mars and Jupiter there have been discovered within the present century no less than twenty-three planetary bodies of extremely small size, of which the accompanying table exhibits certain particulars :

their revolution round the sun in periods varying from 1200 to 1600 days: the orbits of several of them are much inclined to the ecliptic, in which they differ from the old planets; thus-that of Hebe has an inclination of 15°, of Vesta 70°, of Pallas 34°; of Iris, however, only 5o.

124. It will be observed that of these twenty-three, nineteen have been discovered within the last seven years they indicate the investigating spirit of the age and the improvement of instrumental appliances. Nor let it be supposed that chance has had any thing to do with these discoveries. Mr. Hind, the skilful conductor of Mr. Bishop's Observatory in the Regent's Park, has for some time been employed in the accurate construction of a series of charts of that portion of the heavens extending 3° on each side the ecliptic, in which is inserted every star from the 1st to the 11th magnitude. The Berlin charts are on the same plan; and Mr. Cooper of Markree Castle, county Sligo, with the assistance of his astronomer Mr. Graham, is following in the same track. Now, if in the progress of mapping down these minute objects from actual admeasurement, one of them is suspected of having changed its place, it is carefully scrutinised, its distance from its neighbours taken, and at short intervals repeated if the lately-measured distance is found to differ from the former, such object is immediately claimed as a planet, and its discovery announced to the world; observers persecute it, calculation founded on observation lays down its orbit, declares its periodic time and its distance from the sun, and it is forthwith enrolled as a member of the great solar family. The astronomer, encouraged by success, renews his patient

and laborious search. Mr. Hind's announcement of the discovery of Irene shews the promptness and decision with which these small stars are seized upon when suspicion is excited. "On May 19th, 16h 39m, the new star followed a known star by 8s-3, and at 16h 52m the difference of right ascension was 75.7; quite sufficient,” says he, "to establish motion in one object. At 17h 15m the difference of right ascension was 6s-6. The planetary nature of the stranger was therefore satisfactorily proved from 36m interval." The next morning's Times announced the discovery to the world. Such promptitude is necessary to establish the claim of priority, for the same planet was detected by De Gasparis four days later; while Psyche, De Gasparis' planet, discovered on the 17th by him, was searched for as a planet by Hind on the 18th of March, 1852; Mr. Hind having missed it from the place which it had occupied a short time previously.

The immense and disproportionate space between the orbits of Mars and Jupiter was remarked by Kepler; and the existence of these insignificant planets has given rise to a magnificent conjecture—namely, that between the orbits of Mars and Jupiter there once revolved a planet, which, by some sudden convulsion, burst asunder, and that these are the fragments of that exploded globe. An opinion (considered by Mr. Adams unfounded,) was held, that the positions and intersections of their orbits are such as would be required by the laws of mechanics, were a massive globe to explode in the manner supposed. Sir John Herschel's observations must be subjoined, although the French astronomers would seem to receive the conjecture as an

established truth: "This may serve as a specimen of the dreams in which astronomers, like other speculators, occasionally and harmlessly indulge." The application of Bode's law of the planetary distances would, nevertheless, seem to favour the conclusion.

125. The asteroids form a group by themselves. With our powers of artificial vision-supposing the optical organs of the inhabitants, if any exist in them, to resemble those of human beings-it is probable that when one of these small bodies passes by another, the mode of being of its inmates might be disclosed, their towns and fields traced out, and their artificial erections examined, as on a plan or map.

Great discrepancies exist among writers as to the diameters of the asteroids: the probability seems to be, that the largest is the size of the moon; the smallest about 300 miles in circumference. least has an extensive atmosphere.

One of them at

SECTION XII.

ON COMETS.

FORM OF THEIR ORBITS-HOW MAY THE IDENTITY OF A COMET BE PROVED?-THE THREE KNOWN COMETS-PHYSICAL CONSTITUTION -DO THEY MOVE THROUGH AN EXTREMELY SUBTLE MEDIUM, OR THROUGH EMPTY SPACE?

126. COMETS form an entirely distinct class of bodies from either the planets or the fixed stars, whether we regard the character of their movements or their physical constitution.

The word comet is derived from the Latin and Greek word "coma," hair, these bodies being usually distinguished by a train or tail, which was supposed to bear some resemblance to a tuft or lock of hair. They revolve round the sun in orbits either of an elliptical or parabolic form. Sir Isaac Newton supplied us with a proof that any body revolving round another, and kept in its orbit by gravity, would describe in its revolution one of the conic sections; either an ellipse, a parabola, or an hyperbola. It has not yet been determined whether or not the orbit of any comet is an hyperbola; indeed, the great difficulty of calculating the orbits of comets arises from their being seen only when in the immediate neighbourhood of the sun; and during the small space of time they are, for the most part, visible, the three curves differ so little from each other, that the nicest observations are necessary to determine to which the orbit of a given comet belongs.

127. The properties of the ellipse will be pointed out under the head of physical astronomy; those of the parabola will be understood from what follows. Let s, fig. 17, be the place of the sun in the focus, R the place of the comet in its perihelion, and let R Q be taken equal to s R; draw N N perpendicular to s Q. If a comet, P, move in such a manner that its distance, ps, from the sun is always equal to PN, its perpendicular distance from the line N N N, it will describe a parabola of which s is one focus, while the other is at an infinite distance. If such be the nature of its orbit, the orbit will not return into itself, but the comet, after one appearance, will not be seen again.

Now, most of the comets are only visible to us