helion, reascend far above all human vision. Others again are dashing through the solar system, in all possible directions, apparently without any prescribed path, or any guide to direct them in their eccentric wanderings. Instead of revolving uniformly from east to west, like the planets, their motions are direct, retrograde, and in every conceivable direction.

It is remarked by a late writer, that the average inclinations of all the planes in which the comets now on record have been found to move, is about 90°. This he regards as a wonderful instance of the goodness of Providence, in causing their motions to be performed in a manner least likely to come in contact with the earth and the other planets.

307. Of the physical nature of comets, little is known. That they are, in general, very light and vapory bodies, is evident from the fact that stars have sometimes been seen even through their densest portions, and are generally visible through their tails, and from the little attractive influence they exert upon the planets in causing perturbations. While Jupiter and Saturn often retard and delay comets for months in their periodic revolutions, comets have not power, in turn, to hasten the time of the planets for a single hour; showing conclusively that the relative masses of the comets and planets are almost infinitely disproportionate.

Such is the extreme lightness or tenuity of cometary bodies, that in all probability the entire mass of the largest of them, if condensed to a solid substance, wou d not amount to more than a few hundred pounds. Sir Isaac Newton was of opinion, that if the tail of the largest comet was compressed within the space of a cubic inch, it would not then be as dense as atmospheric air! The comet of 1770 got entangled, by attrac tion, among the moons of Jupiter, on its way to the sun, and remained near them for four montis: yet it did not sensibly affect Jupiter or his moons. In this way the orbite of comets are often entirely changed.

308. Comets were formerly regarded as harbingers of famine, pestilence, war, and other dire calamities. In one or two instances, they have excited serious apprehension that the day of judgment was at hand, and that they were the appointed messengers of Divine wrath, hasting apace to burn up the world. A little reflection, however, will show that all such fears are groundless. The same unerring hand that guides the ponderous planet

307. Physical nature of comets? What proofs of their light and vapory character? (What said of their probable mass? Opinion of Newton? What said of the comet of 1770 ? What effect on orbits?)

808. How comets formerly regarded? Why no fears of collision estimate of chances?")

(What

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.

Even those who have calculated the "chances" of collision-as if chance had any thing to do among the solar bodies-have concluded the chances of collision are about as one to 281,000,000-i. e., like the chance one would have in a lottery, where there were 281,000,000 black balls, and but one white one; and where the white ball must be produced at the first drawing to secure a prize.

309. Were a collision actually to take place between a comet and the earth, it is not probable that the former would even penetrate our atmosphere, much less dash the world to pieces. Prof. Olmsted is of opinion that in such an event, not a particle of the comet would reach the earth-that the portions encountered by her would be arrested by the atmosphere, and probably inflamed; and that they would perhaps exhibit, on a more magnificent scale than was ever before observed, the phenomena of shooting stars or meteoric showers. The idea, therefore, that comets are dangerous visitants to our system, has more support from superstition than from reason or science.

The air is to us what the waters are to fish. Some fish swim around in the deep, while others, like lobsters and oysters, keep on the bottom. So birds wing the air, while men and beasts are the "lobsters" that crawl around on the bottom. Now there is no more probability that a comet would pass through the atmosphere, and injure us upon the earth, than there is that a handful of fog or vapor thrown down upon the sur face of the ocean, would pass through and kill the shell-fish at the bottom.

310. After all that is supposed to be known respecting comets, it must be admitted that they are less understood than any other bodies belonging to our system. "What regions these bodies visit, when they pass beyond the limits of our view; upon what errands they come, when they again revisit the central parts of our system; what is the difference between their physical constitution and that of the sun and planets; and what important ends they are destined to accomplish in the economy of the universe, are inquiries which naturally arise in the mind, but which surpass the limited powers of the human understanding at present to determine."

309. What probable effect in case of collision? Prof. Olmsted's opinion? (Remark respecting the air, fish, lobsters, &c. ?)

810. Are we as well acquainted with comets as with other bodies of our system? What inquiries suggested? How answered ?

CHAPTER X.

OF THE SUN.

311. Or all the celestial objects with which we are acquainted, none make so strong and universal an impression upon our globe as does the sun. He is the great center of the solar system-a vast and fiery orb, kindled by the Almighty on the morn of creation, to cheer the dark abyss, and to pour his radiance upon surrounding worlds. Compared with him, all the solar bodies are of inconsiderable dimensions; and without him, they would be wrapped in the gloom of interminable night.

312. The form of the sun is that of an oblate spheroid, his equatorial being somewhat greater than his polar diameter. The mean of the two is 886,000 miles. He is 1400,000 times as large as the mighty globe we inhabit, and 300 times as large as all the planets put together. Were he placed

where the earth is, he would fill all the orbit of the moon, and extend 200,000 miles beyond it in every direction. It would take 112 such worlds. as ours, if laid side by side, to reach across his vast diameter.

1. The vast magnitude of the sun may be inferred from the fact, that when rising or setting, he often appears larger than the largest building, or the tops of the largest trees. Now if the angle filled by him at the distance of two miles is over 100 feet across, what must it be at the distance of 95 millions of miles?

THE SUN AND THE MOON'S ORBIT.

2. Were a railroad passed through the sun's center, and should a train of cars start from one side, and proceed on at the rate of 30 miles an hour, it would require 8 years

811. Describe the sun. How compare with the rest of the system? 812. What is his form? Diameter? Mass, as compared with our globe? With all other bodies of the system? With moon's orbit? (What sensible evidence of the vast magnitude of the sun? Illustration from railroad? Demonstration as to its comparison with moon's orbit?)

to cross over uns diameter. To traverse his vast circumference, at the same rate of speed, would require nearly 11 years.

3. The mean distance of the moon from the earth's center is 240,000 miles; consequently the diameter of her orbit, which is twice the radius, is 480,000. Subtract this from 886,000, the sun's diameter, and we have 406,000 miles left, or 203,000 miles on each side, beyond the moon's orbit.

SPOTS ON THE SUN.

313. By the aid of telescopes, a variety of spots have been discovered upon the sun's disk. Their number is exceedingly variable at different times. From 1611 to 1629, a period of 18 years, the sun was never found clear of spots, except for a few days in December, 1624. At other times, twenty or thirty were frequently seen at once; and at one period, in 1825, upwards of fifty were to be seen. Prof. Olmsted states that over 100 are sometimes visible. From 1650 to 1670, a period of 20 years, scarcely any spots were visible; and for eight years, from 1676 to 1684, no spots whatever

For

were to be seen. For the last 46 years, a greater or less number of spots have been visible every year. several days, during the latter part of September, 1846, we could count sixteen of these spots, which were distinctly visible, and most of them well defined; but on the 7th of October following, only six small spots were visible, though the same telescope was used, and circumstances were equally favorable.

The sun is a difficult object to view through a telescope, even when the eye is protected in the best manner by colored glasses. In some cases (as in one related to the author by Professor Caswell, of Brown University), the heat becomes so great as to spoil the eye-pieces of the instrument, and sometimes the eye of the observer is irrepa rably injured.

314. The solar spots are all found within a zone 60° wide-i. e., 30° each side of the sun's equator. They are generally permanent, though they have been known to

Number of spots seen? 1629? In 1825? Prof. From 1676 to 1684? I1.

813. View of sun's surface through telescopes? Are they always to be seen? How from 1611 to Olmsted's statement? How from 1650 to 1670? 1846 (What said of difficulties of observing?) 814,- Where are these spots situated? Are they permanent? What mo

break in pieces, and disappear in a very short time. They sometimes break out again in the same places, or where none were perceptible before. They pass from left to right over the sun's disk in 13 days, 15 hours, and 45 minutes; from which it has been ascertained that he performs a sidereal revolution on his axis, from west to east, or in the direction of all all

the

planets, every 25 days, 7 hours, and 48 minutes.

1. His apparent or synodic revolution requires 27 days 7 hours; but this is as much more than a complete revolution upon his axis, as the earth has advanced in her orbit in 25 days 8 hours. Let S represent the sun, and A the earth in her orbit. When she is at A, a spot is seen upon the disk of the sun at B.

*

SIDEREAL AND SYNODIC REVOLUTIONS OF THE SUN,

The sun revolves in the direction of the arrows, and in 25 days 10 hours the spot comes round to B again, or opposite the star E. This is a sidereal revolution.

2. During these 25 days 8 hours, the earth has passed on in her orbit some 25°, or nearly to C, which will require nearly two days for the spot at B to get directly toward the earth, as shown at D. This last is a synodic revolution. It consists of one complete revolution of the sun upon his axis, and about 270 over.

315. Of the nature of these wonderful spots, a variety of opinions have prevailed, and many curious theories have been constructed. Lalande, as cited by Herschel, suggests that they are the tops of mountains on the sun's surface, laid bare by fluctuations in his luminous atmosphere; and that the penumbræ are the shoaling declivities of the mountains, where the luminous fluid is less deep. Another gentleman, of some astronomical knowledge, supposes that the tops of the solar mountains are exposed by tides in the sun's atmosphere, produced by planetary attraction.

To the theory of Lalande, Dr. Herschel objects that it is contradicted by the sharp termination of both the internal and external edges of the penumbra; and ad vances as a more probable theory, that "they are the

tion have they? What conclusion from it? (What revolution is this? What time required for a synodic revolution? Illustrate.) 315. What are these spots supposed to be? Lalande ? &c. Dr. Herschel'e emark? Prof Olmsted? Prof. Wilson? Experiments of Prof. Henry?