approaching the Sun. As the comets pass between us and the fixed stars, their envelopes and tails are so thin, that stars of very small magnitude may be seen through them. Some comets, having no nucleus, are transparent throughout their whole extent.

528. The nucleus of a comet sometimes appears opaque, and it then resembles a planet. Astronomers, however, are not agreed upon this point. Some affirm that the nucleus is always transparent, and that comets are in fact nothing but a mass of vapor, more or less condensed at the center. By others it is maintained that the nucleus is sometimes solid and opaque. It seems probable, however, that there are three classes of comets, viz.; 1st. Those which have no nucleus, being transparent throughout their whole extent; 2d. Those which have a transparent nucleus; and, 3d. Those having a nucleus which is solid and opaque.

529. A comet, when at a distance from the Sun, viewed through a good telescope, has the appearance of a dense vapor surrounding the nucleus, and sometimes flowing far into the regions of space. As it approaches the Sun, its light becomes more brilliant, till it reaches its perihelion, when its light is more dazzling than that of any other celestial body, the Sun excepted. In this part of its orbit are seen to the best advantage the phenomena of this wonderful body, which has, from remote antiquity, been the specter of alarm and terror.

530. The luminous train of a comet usually follows it, as it approaches the Sun, and goes before it, when the comet recedes from the Sun; sometimes the tail is considerably curved towards the region to which the comet is tending, and in some instances, it has been observed to form a right angle with a line drawn from the Sun through the center of the comet. The tail of the comet of 1744, formed nearly a quarter of a circle; that of 1689 was curved like a Turkish sabre. (Map IX., Fig. 73.) Sometimes the same comet has several tails. That of 1744 had, at one time, no less than six, which appeared and disappeared in a few days. (See Map IX., Fig. 74.) The comet of 1823 had, for several days, two tails; one extending towards the Sun, and the other in the opposite direction.

531. Comets, in passing among and near the planets, are materially drawn aside from their courses, and in some cases have their orbits entirely changed. This is remarkably true in regard

528. What difference of opinion respecting the nucleus of comets? What probable solution? 529. How do they appear when viewed through a telescope at a distance from the Sun? As it approaches him? Where seen to best advantage? 530. Usual direction of the trains of comets? Other positions? Comet of 1744? Of 1689? Of 1823? 531. Influence of attraction upon comets? Illustrations? Comet of 1770?

to Jupiter, which seems by some strange fatality to be constantly in their way, and to serve as a perpetual stumbling-block to them.

"The remarkable comet of 1770, which was found by Lexell to revolve in a moderate ellipse, in a period of about five years, actually got entangled among the satellites of Jupiter, and thrown out of its orbit by the attractions of that planet," and has not been heard of since.-Herschel, p. 310. By this extraordinary rencontre, the motions of Jupiter's satellites suffered not the least perceptible derangement; a sufficient proof of the aeriform nature of the comet's mass.

532. It is clear from observation, that comets contain very little matter. For they produce little or no effect on the motion of the planets when passing near those bodies; it is said that a comet, in 1454, eclipsed the Moon; so that it must have been very near the Earth; yet no sensible effect was observed to be produced by this cause, upon the motion of the Earth or the Moon.

The observations of philosophers upon comets, have as yet detected nothing of their nature. Tycho Brahe and Appian supposed their tails to be produced by the rays of the Sun transmitted through the nucleus, which they supposed to be transparent, and to operate as a lens. Kepler thought they were occasioned by the atmosphere of the comet, driven off by the impulse of the Sun's rays. This opinion, with some modification, was also maintained by Euler. Sir Isaac Newton conjectured that they were a thin vapor, rising from the heated nucleus, as smoke ascends from the Earth; while Dr. Hamilton supposed them to be streams of electricity.

"That the luminous part of a comet," says Sir John Herschel, "is something in the nature of a smoke, fog, or cloud, suspended in a transparent atmosphere, is evident from a fact which has been often noticed-viz., that the portion of the tail where it comes up to, and surrounds the head, is yet separated from it by an interval less luminous; as we often see one layer of clouds laid over another with a considerable clear space between them." And again: "It follows that these can only be regarded as great masses of thin vapor, susceptible of being penetrated through their whole substance by the sunbeams."

533. Comets have always been considered by the ignorant and superstitious, as the harbingers of war, pestilence, and famine. Nor has this opinion been, even to this day, confined to the unlearned. It was once universal. And when we examine the dimensions and appearances of some of these bodies, we cease to wonder that they produced universal alarm.

According to the testimony of the early writers, a comet which could be seen in daylight with the naked eye, made its appearance 43 years before the birth of our Saviour. This date was just after the death of Cæsar, and by the Romans, the comet was believed to be his metamorphosed soul, armed with fire and vengeance. This comet is again mentioned as appearing in 1106, and then resembling the Sun in brightness, being of a great size, and having an immense tail. In the year 1402, a comet was seen, so brilliant as to be discerned at noon-day.

534. In 1456, a large comet made its appearance. It spread a wider terror than was ever known before. The belief was very general, among all classes, that the comet would destroy the Earth, and that the Day of Judgment was at hand!

582. What said of their physical natures? Opinion of Tycho Brahe? Of Kepler and Euler? Of Newton and Dr. Hamilton? Of Sir John Herschel? 533. How have comets usually been regarded by the ignorant? What remarkable comet mentioned? 534. What comet in 1456 ? Effect of its appearance? Has it appeared since? Its period?

The same comet appeared again in the years 1531, 1607, 1682, 1758 and 1835. It passed its perihelion in November, 1835, and will re-appear every 75 years thereafter.

At the time of the appearance of this comet, the Turks extended their victorious arms across the Hellespont, and seemed destined to overrun all Europe. This added not a little to the general gloom. Under all these impressions, the people seemed totally regardless of the present, and anxious only for the future. The Romish Church held at this time unbounded sway over the lives, and fortunes, and consciences of men. To prepare the world for its expected doom, Pope Calixtus III. ordered the Ave Maria to be repeated three times a day, instead of two. He ordered the church bells to be rung at noon, which was the origin of that practice, so universal in Christian churches. To the Ave Maria, the prayer was added-"Lord! save us from the Devil, the Turk and the Comet;" and once, each day, these three obnoxious personages suffered a regular excommuni

cation.

The Pope and clergy exhibiting such fear, it is not a matter of wonder that it became the ruling passion of the multitude. The churches and convents were crowded for confession of sins; and treasures uncounted were poured into the Apostolic chamber.

The comet, after suffering some months of daily cursing and excommunication, began to show signs of retreat, and soon disappeared from those eyes in which it found no favor. Joy and tranquillity soon returned to the faithful subjects of the Pope, but not so their money and lands. The people, however, became satisfied that their lives, and the safety of the world, had been cheaply purchased. The Pope, who had achieved so signal a victory over the monster of the sky, had checked the progress of the Turk, and kept, for the present, his Satanic majesty at a safe distance; while the Church of Rome, retaining her unbounded wealth, was enabled to continue that influence over her followers, which she retains, in part, to this day.

535. The comet of 1680 would have been still more alarming than that of 1456, had not science robbed it of its terrors, and history pointed to the signal failure of its predecessor. This comet was of the largest size, and had a tail whose enormous length was more than ninety-six millions of miles. (Map IX., Fig. 75.)

At its greatest distance, it is 13,000,000,000 of miles from the Sun; and at its nearest approach, only 574,000 miles from his center; or about 130,000 miles from his surface. In that

*

* In Brewster's edition of Ferguson, this distance is stated as only 49,000 miles. This is evidently a mistake; for if the comet approached the Sun's center within 49,000 miles, it would penetrate 390,000 miles below the surface! Taking Ferguson's own elements for computing the perihelion distance, the result will be 494,460 miles. The mistake may be accounted for, by supposing that the cypher had been omitted in the copy, and the period pointed off one figure farther to the left. Yet, with this alteration, it would be still incorrect; because the Earth's mean distance from the Sun, which is the integer of this calculation, is assumed at 82,000,000 of miles. The ratio of the comet's perihelion distance from the Sun, to the Earth's mean distance, as given by M. Pingré, is as 0.00603 to 1. This multiplied into 95,273,869, gives 574,500 miles for the comet's perihelion distance from the Sun's center; from which, if we substract his semi-diameter, 443,840 miles, we shall have 130,660 miles, the distance of the comet from the surface of the Sun.

Again, if we divide the Earth's mean distance from the Sun, by the comet's perihelion distance, we shall find that the latter is only 1-166th part of the Earth's distance. Now the square of 166 is 27,556; and this expresses the number of times that the Sun appears larger to the comet, in the above situation, than it does to the Earth. Squire makes it 34,596 times larger.

According to Newton, the velocity is 880,000 miles per hour. More recent discoveries indicate a velocity of 1,240,108 miles per hour,

Incidents respecting the Turks and Church of Rome? 535. Comet of 1680? Length of its tail? Aphelion and perihelion distances? Rapidity of its motion when nearest the Sun ? What error corrected? Appearance of the Sun from that point? Heat of the comet? Indicates what? Fanciful theory of Dr. Whiston, and remarks upon it?

part of its orbit which is nearest the Sun, it flies with the amazing swiftness of 1,000,000 miles in an hour, and the Sun, as seen from it, appears 27,000 times larger than it appears to us; consequently, it is then exposed to a heat 27,000 times greater than the solar heat at the Earth. This intensity of heat exceeds, several thousand times, that of red-hot iron, and indeed all the degrees of heat that we are able to produce. A simple mass of vapor, exposed to a thousandth part of such a heat, would be at once dissipated in space--a pretty strong indication that, however volatile are the elements of which comets are composed, they are, nevertheless, capable of enduring an inconceivable intensity of both heat and cold.

This is the comet which, according to the reveries of Dr. Whiston and others, deluged the world in the time of Noah. Whiston was the friend and successor of Newton; but, anxious to know more than is revealed, he passed the bounds of sober philosophy, and presumed not only to fix the residence of the damned, but also the nature of their punishment. According to this theory, a comet was the awful prison-house in which, as it wheeled from the remotest regions of darkness and cold into the very vicinity of the Sun, hurrying its wretched tenants to the extremes of perishing cold and devouring fire, the Almighty was to dispense the severities of his justice. Such theories may be ingenious, but they have no basis of facts to rest upon. They more properly belong to the chimeras of Astrology, than to the science of Astronomy.

536. When we are told by philosophers of great caution and high reputation, that the fiery train of the comet, just alluded to, extended from the horizon to the zenith; and that that of 1744 had, at one time, six tails, each 6,000,000 of miles long, long, and that another, which appeared soon after, had one 40,000,000 of miles long, and when we consider also the inconceivable velocity with which they speed their flight through the solar system, we may cease to wonder if, in the darker ages, they have been regarded as evil omens.

But these idle fantasies are not peculiar to any age or country. Even in our own times, the beautiful comet of 1811, the most splendid one of modern times, was generally considered among the superstitious, as the dread harbinger of the war which was declared in the following spring. It is well known that an indefinite apprehension of a more dreadful catastrophe lately pervaded both continents, in anticipation of Biela's comet of 1832.

537. The nucleus of the comet of 1811, according to observations made near Boston, was 2617 miles in diameter, corresponding nearly to the size of the Moon. The brilliancy with which it shone, was equal to one-tenth of that of the Moon. The envelope, or aeriform covering surrounding the nucleus, was 24,000 miles thick, about five hundred times as thick as the atmosphere which encircles the Earth; making the diameter of comet, including its envelope, 50,617 miles. It had a very

536. Why not strange that these comets were regarded as evil omens? Are such superstitions peculiar to any age or country? What illustrations? 587. Size of the comes of 1811? Its motion at its perihelion ?

luminous tail, whose greatest length was one hundred millions of miles. Map IX., Fig. 76. This comet moved, in its perihelion, with an almost inconceivable velocity-fifteen hundred times greater than that of a ball bursting from the mouth of a cannon.

538. According to Regiomontanus, the comet of 1472 moved over an arc of 120° in one day. Brydone observed a comet at Palermo in 1770, which passed through 50° of a great circle in the heavens in 24 hours. Another comet, which appeared in 1759, passed over 41° in the same time. The conjecture of Dr. Halley, therefore, seems highly probable, that if a body of such a size, having any considerable density, and moving with such a velocity, were to strike our Earth, it would instantly reduce it to chaos, mingling its elements in ruin.

The transient effect of a body passing near the Earth, could scarcely amount to any great convulsion, says Dr. Brewster; but if the Earth were actually to receive a shock from one of these bodies, "having any considerable density," the consequences would indeed be awful. A new direction would be given to its rotary motion, and it would revolve around a new axis. The seas, forsaking their beds, would be hurried, by their centrifugal force, to the new equatorial regions; islands and continents, the abodes of men and animals, would be covered by the universal rush of the waters to the new equator, and every vestige of human industry and genius would be at once destroyed. But so far as we are as yet acquainted with these singular bodies, they are altogether too light and gasseous to produce any such results by collision.

539. The chances against such an event, however, are so very numerous, that there is no reason to dread its occurrence. The French government, not long since, called the attention of some of her ablest mathematicians and astronomers to the solution of this problem; that is, to determine upon mathematical principles, how many chances of collision the Earth was exposed to. After a mature examination, they reported-" We have found that, of 281,000,000 of chances, there is only one unfavorable-there exists but one which can produce a collision between the two bodies." "Admitting, then," say they," for a moment, that the comets which may strike the Earth with their nucleuses, would annihilate the whole human race; the danger of death to each individual, resulting from the appearance of an unknown comet, would be exactly equal to the risk he would run, if in an urn there was only one single white ball among a total number of 281,000,000 balls, and that his condemnation to death would be the inevitable consequence of the white ball being produced at the first drawing."

A little reflection, however, will show that all such fears are groundless. The same unerring hand that guides the ponderous planet in its way, directs also the majestic comet; and where infinite wisdom and almighty power direct, it is almost profane to talk of collision or accident.

540. We have before stated that comets, unlike the planets, observe no one direction in their orbits, but approach to, and recede from their great center of attraction, in every possible

538. Velocity of the comet of 1472? Of 1770? Of 1759? Dr. Halley's conjecture? Dr. Brewster's? Could a comet produce any such effects? 539. Is such a collision probable? Why not? 540. What said of the orbits of comets and their various directions?