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Calculation Of A Comet's Eeturn.—As we can observe so small a proportion of the entire orbit, it is very difficult, indeed oftentimes impossible, to decide whether it is an ellipse, hyperbola, or parabola. A few are known to move in clearly elliptical paths, and their movements have been so accurately estimated that it is possible to predict their exact place in the starry vault on any given day and hour. The other comets may never return, or at least not for centuries hence. They may be paying our sun their first visit; or if they have swept through the solar system before, it may have been at so remote a time that no record is preserved, even if it were not before the creation of man. Under these circumstances it is obviously extremely difficult to determine the times of these apparently erratic wanderers; yet, in spite of all these obstacles, some have been tracked far into space beyond the telescopic view. For example, the comet of 1844 is announced to pay a visit to the astronomers of the year of our Lord 101,844. The period of the comet of 1744, is fixed at 122,683 years.

Distance From The Sun.—The comets at their perihelion sweep very near the sun. Thus the one of 1680 came where the temperature was estimated by Newton to be about 2,000 times that of red-hot iron. The nearest approach known is that of the comet of 1843, whose perihelion distance was but about 30,000 miles from the surface of the sun; in fact, it doubled around that body in two hours' time. (Guillemin.) The greatest aphelion distance yet estimated is that of the comet of 1844, which is over 400,000,000,000 miles. The velocity varies, of course, with the position in the orbit. The comet of 1680 moved in perihelion at the rate of over two hundred and seventy-seven miles per second; while in aphelion its velocity is only about six miles per hour.

Density Of Comets.—The quantity of matter contained in a comet is exceedingly small. Telescopic stars even are visible through them. The comet of 1770 became entangled among Jupiter's moons, and remained there four months without interfering with their movements in the least; indeed, so far from that, its own orbit was so much changed by the proximity, that from a periodical return of 5^ years, it has not been seen since. The same comet came within 1,400,000 miles of the earth without producing any sensible effect. In 1861, we have good reason to suppose that the earth actually passed through the tail of a comet, its presence being indicated only by a peculiar phosphorescent mist. So that even should our earth run full-tilt against a comet, the shock would be quite imperceptible.* Still, however lightly we may speak of the probability of such a collision, we must remember that there are comets of greater solidity. Donati's, for instance, is estimated to be about -^ the bulk of the earth. The concussion of such a body, moving

* " However dangerous might be the shock of a comet, it might be so slight that it would only do damage at that part of the earth where it actually struck; perhaps even we might cry quits, if, while one kingdom were devastated, the rest of the earth were to enjoy the rarities which a body coming from so far might bring to it. Perhaps we should be very surprised to find that the dibris of these masses that we despised were formed of gold or diamonds; but who would be the more astonished—we or the comet-dwellers who would be cast on our earth? What strange beings each would find the other!" Lettre »ir la GomHe—(M De Maupertuis.)

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

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