from its luminous mantle, when that mantle has so much the appearance of flame, from which heat is generally diffused on the earth? But investigations into the nature of the Sun must be attended with so much uncertainty, that perhaps no theory on the subject can be free from objection.

Much light has been thrown upon heat or caloric by the improvements of modern chemistry. But satisfactory conclusions concerning its nature cannot be drawn. Lord Bacon considered heat "the effect of an intestine motion, or mutual collision of the particles of the body heated, an expansive undulatory motion in the minute parts of the body." Count Rumford's experiments seemed to show, that caloric "was imponderable, and capable of being produced ad infinitum from a finite quantity of matter.' "He concluded, that "it must be an effect arising from some species of corpuscular action amongst the constituent parts of the body." Other chemists consider it "an elastic fluid sui generis."*

Mr. Dick, a Scotch author of much ingenuity, in his Christian Philosopher, has a note on the planet Mercury, deserving consideration. "From a variety of facts, which have been observed in relation to the production of caloric, it does not appear probable, that the degree of heat on the surfaces of different planets is inversely proportional to the square of their respective distances from the Sun. It is more probable, that it depends chiefly on the distribution of the substance of caloric on the surfaces, and throughout the atmospheres of these bodies, in different quantities, according to the different situations they occupy in the solar system; and that these different quantities of caloric are put into action by the influence of the solar rays, so as to produce that degree of sensible heat requisite for each respective planetary globe. On this hypothesis, which is corroborated by a great variety of facts and experiments, there may be no more sensible heat felt on the surface of the planet

* Of its own kind.

Mercury, than on the surface of Herschel, although one of these bodies is nearly fifty times nearer the Sun than the other. We have only to suppose, that a small quantity of caloric exists in Mercury, and a larger quantity in Herschel, proportionate to his distance from the centre of the system. On this ground, we have no reason to believe, either that the planets nearest the Sun are parched with excessive heat, or that those that are most distant are exposed to all the rigor of insufferable cold; or that the different degrees of temperature which may be found in these bodies, render them unfit for being the abodes of sensitive and intellectual beings."

This theory of caloric is modern and popular; but, like others on the same subject, does not command unqualified assent. If heat be a fluid only, why is it radiated by all bodies? and why, reflected, does it pass from object to object in rays, a manner so dissimilar to the movement of other fluids? It may be that the learned world must be content, as in attraction, with knowing the operations of heat, without being able to investigate its nature.

Any uncertainty respecting caloric, must rest on the physical construction of the Sun, the prime agent of heat in whatever way produced. From what has been said of solar clouds, it must be apparent that some authors consider the Sun surrounded by an atmosphere of vast extent. They ground their opinion principally on the authority of Dr. Herschel, supported by his observations. "The height of the atmosphere he computes to be not less than eighteen hundred forty-three, nor more than two thousand seven hundred sixty-five miles, consisting of two regions; that nearest the Sun being opaque, and probably resembling the clouds of our Earth; the outermost emitting vast quantities of light, and forming the apparent luminous globe we behold."

Harriot, an Englishman, or Fabricius, a German, first discovered the spots on the Sun, about the year 1610. According to some authors, they were first seen by Galileo, or Scheiner. An account of his observations on

them was published by Fabricius in 1611. The spots are various in shape and magnitude. Some have been observed large enough to cover the whole eastern continent, Europe, Asia, and Africa; some to cover the surface of the whole Earth; and one was observed by Dr. Herschel, in 1799, computed to be more than fifty thousand miles in diameter. In most of them, there is a very dark nucleus, surrounded by an umbra, or fainter shade. A distinct and well-defined boundary intervenes between the umbra and nucleus. The part of the umbra nearest the dark nucleus is generally brighter than that portion which is more distant.

A spot on the Sun appears at the Earth to perform a revolution round the Sun from west to east in a little more than twenty-seven days, a period longer than the time in which the Sun revolves on its axis. The excess is occasioned by the motion of the Earth in its orbit. The spots on the Sun are generally confined to a zone extending about 35° each way from the solar equator. None have been seen nearer the poles than the solar latitude of 39o 5'.

The Sun rarely appears pure and unsullied by spots. Sometimes, however, none are seen on his disk for several years in succession. From the year 1676 to the year 1684, not a single spot was seen on the Sun.

The light of the Sun is progressive, and not instantaneous, as formerly supposed. It is a little more than eight minutes in coming from the Sun to the Earth. On this account, the Sun and other heavenly bodies appear to the east of their true place. Let S be the Sun, (Plate v. Fig. 4,) A B C the equator, or a parallel of latitude on the Earth. If light were instantaneous, it would be noon at A, when the Sun is on the meridian, as at D. But as light is progressive, a meridian must pass more than two degrees eastward from A to B, after a ray is emitted from the Sun, before it arrives at the Earth. The Sun, when over the meridian at A, must appear at E. The student may think it more truly Copernican to be told, that light, emitted from the Sun

towards a spectator, does not arrive at him, but presents the image of that luminary to inhabitants two degrees to the west of his meridian.

What is the Sun? How is the Sun placed in respect to the planetary orbits? Has the Sun any motion? How often does it turn on its axis? How is it proved that the Sun is globular? What is its diameter? Its circumference? How many times larger than the Earth is the Sun? How long would a celestial courier, passing at the rate of 40 miles in a day, be in circumambulating the Sun? Would 240,000 miles, the distance of the Moon from the Earth, reach from the centre to the circumference of the Sun? What was the ancient opinion respecting the physical construction of the Sun? What must the surface of the Sun resemble? If heat come from the Sun, why is it always cold in the upper regions of the air? Does the height of he most elevated mountain, or of the atmosphere, bear any perceptible proportion to the distance of the Sun from the Earth? According to Dr. Herschel's opinion, are there spots on the Sun? What was Dr. Herschel's hypothesis respecting the Sun? What are openings? What are shallows? What are ridges? What are nodules? What are corrugations? What are indentations? What are pores? What was Dr. Brewster's opinion respecting the physical construction of the Sun? What objection can be made to the theories of Dr. Brewster and Dr. Herschel? What were the opinions of different authors respecting heat or caloric? What objection is there to Mr. Dick's theory? Has the Sun an atmosphere? What is considered its height? By whom were spots first discovered on the Sun? At what time were they discovered? How large spots have been seen? Does the Sun ever appear free from spots? Is the light of the Sun instantaneous? In what time does it come from the Sun to us? Do the heavenly bodies appear in their true place?

SECTION III. Of the Planets.

The word planet is derived from the Latin planeta. This is a derivative from the Greek planao, I cause to wander. The Greek primitive planee, error, or wandering, is the root, or original word.

From modern discoveries, the primary planets may now be reckoned eleven-Mercury, Venus, the Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Herschel. All these revolve round the Sun in elliptical orbits from west to east, at different distances and in different times. (Plate i. Fig. 3.)

Eighteen secondary planets, or satellites, have been discovered. One revolves round the Earth; four round Jupiter; seven round Saturn; and six round Herschel. All the primary planets are governed by two great fundamental laws, called, from their discoverer, the great laws of Kepler.

1. If a line be conceived drawn from a planet to the Sun, called a vector radius, such a line would pass over equal areas in equal times.

2. The squares of the periodical times are as the cubes of their mean distances from the Sun.

These are established laws "between the rate of motion in any revolving body, whether primary or seconda-' ry, and its distance from the central body, about which it revolves." They must, therefore, apply to the satellites in revolving around their primaries.

From what is the word planet derived? What are primary planets? How many primary planets are there? What are secondary planets? How many secondary planets have been discovered? Which of the primaries have satellites? How many have each?

SECTION IV. Of Mercury.

Mercury is the planet nearest the Sun. So it is still considered, after the most accurate modern discoveries. It shines by a very brilliant and white light; but the short period in which it can be viewed, and the position of its body seen through the mists of the horizon, have prevented important discoveries being made on its surface. Of all the planets, Mercury is the most swift in its motion. On this account, the name was given to it by the ancients after "the nimble messenger of the gods." It was "represented by the figure of a youth with wings at his head and feet; whence is derived ☀, the character by which it is commonly represented." So great is the velocity of this planet, that it performs more than two revolutions to one of Venus, and, commencing at a conjunction, would pass the Earth three times before it would