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with the speed of a cannon-ball, would undoubtedly produce a very sensible effect.

It is not understood whether comets shine by their own or by reflected light. If, however, their nuclei consist of white-hot matter, a passage through such a furnace would be any thing but desirable or satisfactory. After all the calculations of Astronomy, our only safety lies in that Almighty Power which traces the path and guides the course alike of planets and comets: He, whose eye marks the fall of the sparrow, sees as well the flight of the worlds He has created

Variations In Form And Dimensions.—Comets appear to be subject to constant variations. They are now generally thought to decrease in brilliancy at each successive revolution about the sun. The same comet may present itself sometimes with a tail, and sometimes without. "When the comet first appears, there is generally no tail visible, and the light is faint. As it approaches the sun, however, its brightness increases, the tail shoots out from the coma, and grows daily in length and splendor. Supernumerary tails, shorter and less distinct than the principal one, dart out, but they generally soon disappear, as if from lack of material. The tail of the comet of 1843, just after the perihelion, increased in length 5,000,000 miles per day. As the tail thus extended, the nucleus was correspondingly contracted, so that this comet actually "exhausted itself in the manufacture of its own tail."

Remarkable Comets.—Among the many comets celebrated in history, we shall only notice some of those that have appeared in the present century. The great comet of 1811 was a magnificent spectacle. The head was 112,000 miles in diameter; the nucleus was 400 miles; while the tail, of a beautiful fan-shape, stretched out 112,000,000 miles. The aphelion distance of this comet is fourteen times that of Neptune, or 40,000,000,000 miles. It is announced to return in thirty centuries! To what profound depths of space, beyond the solar system, beyond the reach of the telescope, must such a journey extend!

The comet of 1835 is commonly known as Halley's comet. This is remarkable as being the first comet whose period of revolution was satisfactorily established. Dr. Halley, on examining the accounts of the great comets of 1531, 1607, and 1682, suspected that they were only the reappearance of the same comet, whose period he fixed at about 75 years. He finally ventured to predict the return of the comet about the end of 1758 or beginning of 1759. Although Halley did not live to see his prophecy fulfilled, great interest was felt in the result. It was not destined, however, for a professional astronomer to be the first to detect the comet. A peasant near Dresden saw it on Christmas night, 1758. The history of this comet, as it has been traced back by its period of seventy-five years, is quite eventful. It was seen in England in 1066, when it was looked upon with dread as the forerunner of the victory of William of Normandy. It was then equal to the full moon in size. In 1456, its tail reached from the horizon to the zenith. It was supposed to indicate the success of Mahomet II., who had already taken Constantinople, and threatened the whole Christian world. Pope Calixtus HI., therefore, ordered extra Ave Marias to be repeated by everybody, and also the church bells to be rung daily at noon (whence originated the custom now so universal). A prayer was added as follows: "Lord, save us from the devil, the Turk, and the comet." In 1223, it was considered the precursor of the death of Philip Augustus. The first recorded appearance of Halley's comet was B. 0.130, when it was supposed to herald the birth of Mithridates.

The cornel of 1843 was so intensely brilliant that it was visible in full daylight. It was so near the sun as " almost to graze his surface."

Encke's comet has a period of only 3^ years. A most interesting discovery has been made from observations upon its motion. The comet returns each time to its perihelion about 1\ hours earlier than the most perfect calculations indicate. Hence, Prof. Encke has been led to conjecture that space is filled with a thin, ethereal medium capable of diminishing the centrifugal force, and thus contracting the orbit of a comet.

Donates comet, which appeared in 1858, was the subject of universal wonder. When first discovered, in June, it was 240,000,000 miles from the earth. In August, traces of a tail were noticed, which expanded in October to about 50,000,000 miles in length. This

Fig. 68.



comet, though small, has never been exceeded in the brilliancy of the nucleus and the graceful curvature of the tail. It will return in about 2,000 years.

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Description.—If we watch the western horizon in March or April, just after sunset, we shall sometimes see the short twilight of that season illuminated by

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