liarities recorded four times before its appearance in 1682. In 1305, it is described by the writers of that age as a comet of a dreadful size. In 1456, its train extended through the heavens for the space of 90°, stretching from the horizon to the zenith, and filled all Europe with such terror that, by the decree of the Pope, prayers were of fered in all the Catholic churches, and the bells rung at midday in order to avert the wrath of heaven. In 1682, the tail was only 30° in length; in 1759, it had so diminished in size that it was not visible to the naked eye until it had passed its perihelion, while in 1835 its tail was about 20° in length.

655. ENCKE'S COMET. This comet receives its name from Prof. Encke, of Berlin, who first ascertained that it returned at stated times, in the short period of 1,211 days, or about 3 years. Prof. Encke made this discovery upon its fourth recorded appearance in 1819, and predicted its return in 1822. It came at the appointed time, and from that year forward it has returned at its regular intervals, obeying the same law of gravitation that controls the earth in her orbit.

656. BIELA'S COMET. This is another small cometary body which revolves about the sun in the period of 2,410 days, or about 63 years. This discovery was made by Mr. Biela, of Josephstadt, in the year 1826, who predicted the return of the comet in 1832. The prediction was fulfilled. In 1839, its position was very unfavorable for observation, and there is no record of its having been observed at all at this time.

657. Upon its return in the year 1846, this body was most surprisingly modified, for instead of one comet it was separated into two bodies, each having the true characteristics of a comet. These twin bodies, which were termed the comet and its companion, passed along through the heavens side by side for the space of 70°, changing in their relative brightness and magnitude, and

also in their distances from each other.

658. According to Mr. Plantamour of Geneva, the distance between the nucleus of the comet, and that of its

Give the full account of Encke's, of Biela's?

companion, during the time of their visibility varied from 149,000 miles to 154,000 miles.

659. FAYE'S COMET. Mr. Faye, of the Observatory of Paris, discovered on the 22d of November, 1843, a telescopic comet, which on the 27th of December, was rediscovered in this country by Mr. Joseph S. Hubbard at New Haven. It had a bright nucleus and fan-like tail. It was found to revolve about the sun in an elliptical orbit, in the space of 2,718 days, or a little less than 7 years.

660. The return of the comet to its perihelion was predicted within a day or two of the 3rd of April, 1851. It was seen by Prof. Challis, of Cambridge, England, on the 28th of November, 1850, and was observed by him until the 4th of March, 1851.

661. DE VICO'S COMET. This comet, which was discovered by De Vico, director of the observatory at Rome, on the 22d of August, 1844, possesses a brilliant nucleus and small tail and when most vivid is visible to the naked eye. The calculations of several astronomers soon showed that it revolved in an elliptical orbit, and that the period of revolution was 1,990 days, or nearly 5 years.

662. The date assigned for its return was about the 13th of January, 1850, but there is no record of its reappearance at that time.

663. COMET OF 1680. This remarkable comet, whose surpassing size and splendor we have already alluded to, is supposed with great probability to revolve about the sun in the long period of 575 years.

664. It is regarded as identical with a vast and brilliant comet which was beheld at Constantinople, in the year 1105, A.D., with one that was seen close to the sun in the year 575 A.D., with a third which appeared near the time of the assassination of Julius Cæsar, in the year 43 B.C.: and lastly, with two others mentioned in the Sybilline Oracles, and in Homer, which according to the most reliable calculations were visible in the years 618, and 1194 B.C.

Give the full account of Faye's and of De Vico's? State what is further said of the comet of 1680 ?

665. COMET OF 1843. We have already stated many particulars respecting this most extraordinary body, but a further description is by no means superfluous. It was seen on the 28th of February, 1843, close to the sun, its brightness being so great that the splendor of the solar beams could not overpower its brilliancy.

666. "In New England," says Professor Loomis "it was beheld from half past 7 Á.M., till 3 P.M., when the sky became considerably obscured by clouds. The appearance was that of a luminous globular body; the head of the comet, as observed by the naked eye appearing curcular; its light equal to that of the moon at midnight in a clear sky, and its apparent size about th the area of the full moon."

667. At the Cape of Good Hope, it was seen by every person on board the Owen Glendower, on the day just mentioned, at about sunset, near the sun, and having the shape of a dagger.

668. The vast extent of the tail has already been stated. At the Cape of Good Hope, it appeared on the 3rd of March to be double, two trains diverging from the head in a straight line, forming a small angle with each other. Near the equator this magnificent appendage shone with such a glow that at times it threw a bright light upon the sea. The comet remained visible only for a short time, the earliest observation upon it appears to have been made on the 27th of February, and the latest on the 15th of April.

669. The elements of this comet are not yet decidedly ascertained. A brilliant comet appeared in 1668, the head of which was concealed by the splendor of the solar rays, and whose tail, extending to an immense distance, was so vivid that its image was reflected from the surface of the sea. The investigations of astronomers point in their results to an identity between the comets of 1668 and 1843; inasmuch as on the whole, they present nearly similar aspects, pursue nearly the same path, and the period of revolution assigned to each is 175 years. Prof. Hubbard, of the Washington Observatory, finds however, from a rigorous discussion

Describe in full the comet of 1843 ?

COLLISION WITH THE EARTH.

273

of all the observations made on the comet of 1843, that it most probably revolves in an elliptical orbit, in a period of about 170 years.

670. PHYSICAL NATURE OF COMETS. These extraordinary bodies consist of matter, but existing in an attenuated and diffused state, of which we have no adequate conception. That they consist of matter is proved by the fact that they revolve in regular orbits around the sun, obeying the same law of attraction as the solid masses of the planets; and that this matter is extremely rare and subtile, is shown by the circumstance that the smallest stars are visible through the tail of a comet.

671. A light cloud in comparison with the matter composing the tail of a comet, is to be regarded as a dense and heavy body. For while the former, though gauze-like in its texture and of moderate thickness is yet sufficiently dense to obscure the light of a star; the latter, notwithstanding it is millions of miles in extent, permits the stellar rays to traverse its vast thickness, and to reach the eye, distinctly revealing the orb from which they ema

nate.

672. The amount of matter in comets, even of the largest size, is so small that their passage around the sun has never in the least perceptible degree affected the stability of the solar system; in other words, they have never as far as could be perceived, caused the planets to deviate a hairs' breadth from their accustomed paths around the sun.

673. According to the celebrated La Place, if the mass of the comet of 1770, which passed within 1,500,000 miles of our globe, had been equal to that of the earth, it would have increased our sidereal year by 2h. 53'. But the profound investigations of Delambre, showed that the length of the year was not increased by the fraction of a second, and that consequently the mass of the comet, could not have been equal to one-five thousandth part of the mass of the earth.

674. COLLISION WITH THE EARTH.

Fears have often

State what is said respecting the physical nature of comets? Show that the matter of comets must be very much attenuated? Why must their amount of matter be small? Give the proof?

been entertained that collisions might occur between the earth and comets. When any one of these bodies has its perihelion within the orbit of Mercury it must necessarily cross the orbits of all the planets, and such a collision may possibly take place, but the probability is exceedingly small.

675. Upon the supposition that the nucleus of a comet, possesses a diameter one-fourth the size of that of the earth, and that its perihelion is within the earth's orbit, Arago has computed the chance of our meeting the comet to be as 1 to 281,000,000.

676. But were the earth to meet a comet, it would be somewhat like a cannon ball meeting a cloud, and the earth would probably suffer but little from the encounter. Indeed, it has been supposed by some, that we have already passed through the tail of a comet without knowing it, for, according to Mrs. Somerville, there is reason to think that such was the case when the great comet of 1843, revealed its splendors, to our eyes.

CHAPTER VII.

TIDES.

677. THE periodical rising and falling of the waters of the ocean in alternate succession are called tides. Standing on the sea shore, a person will perceive that for the space of nearly 6 hours the waters of the sea continue to rise higher and higher, overflowing the shores, and running into the channels of the rivers. When they have attained their greatest elevation, it is then said to be high tide, full sea, or flood tide. Remaining at this elevation only for a few moments they then begin to fall, and continue to sink for about 6 hours more. When the waters have reached their greatest depression, it is then low, or ebb tide. After attaining this point, the sea in a short time again

Is it possible for a comet to strike the earth? Is it probable? collision with a comet probably have upon the earth? May we through the tail of a comet? What does Chapter VII. treat of? Describe them, explaining the meaning of high tide and low tide?

What effect would a have already passed What are the tides ?