some approximate idea of the intensity of the solar heat at the surface of the sun. By exposing a surface of ice to the direct action of the sun's heat, when the sun was nearly vertical, Sir John Herschel determined by experiment the thickness of the ice melted in a given time.

From this and like experiments it is determined that it would require the combustion of more than one hundred and thirty thousand pounds of coal per hour on each square foot of the sun's surface to produce a heat equal to that radiated from the solar orb.

When an image of the sun is received on any surface, it is found that the central point of the image is more heated than the parts near the circumference, and that the temperature diminishes from the equator toward the poles.

THE SUN'S ATMOSPHERE.-These facts have been accounted for by supposing the sun to be surrounded by a dense atmosphere, and that the heated rays which pass through the deepest part of this atmosphere, lose a portion of their heat, and hence the regions around the disc of the sun should be, to us, less heated than those near the centre of the solar orb. There are some phenomena attending a total eclipse of the sun which seem to sustain this hypothesis of a solar atmosphere. At the moment the eclipse becomes total, there is seen to burst from the jet-black disc of the moon a sort of halo or glory, radiating on every side, and presenting a spectacle of wonderful grandeur, so much so that on the occasion of the eclipse of July, 1842, witnessed at Pavia,* the entire populace burst into a shout of wonder and admiration.

There also appeared, at the same time, flames of fire darting from behind the limb of the moon, resembling mountains of rose-coloured light, rising to the height of forty or fifty thousand miles above the surface of the sun. These flames are known to assume the form of cloudy exhalations, which, in some instances, seem to be drifted like smoke ascending

* In Lombardy.

in a calm atmosphere to a certain level, where it meets a current and is borne off horizontally.

There is another phenomenon attending the rising and setting of the sun at certain seasons of the year in the shape of a vast beam of faint gauzy light, of lenticular form, rising from the point of sunset in the evening, and stretching upward in the direction of the sun's path sometimes 70° or 80°. This is called the Zodiacal Light, and has long been. regarded as the evidence of uncondensed nebulosity, or a material atmosphere surrounding the equatorial regions of the sun. The central line, or axis, of this luminous beam does not appear to be fixed in position ; and hence a difficulty arises not readily removed by the hypothesis of a material atmosphere.

Some have supposed this mysterious luminous zone to be a nebulous ring surrounding our moon, while others have regarded it as an immense ring of minute asteroids or meteors, revolving round the sun, and slowly subsiding into this grand luminary, and by the conversion of their velocity into heat, as they fall in a perpetual shower on the sun, or are burned up in the solar atmosphere, keeping up a supply equal to the vast radiation shot forth from the sun at every moment of time. While we are willing to admit that a material globe, falling into the solar atmosphere, may generate immense heat, in proportion to its magnitude and velocity, it seems quite impossible to adopt the hypothesis that the zodiacal light is either a material solar atmosphere or a ring of revolving meteors, as it extends to such a vast distance from the sun, that if revolving with the sun, as does our atmosphere with the earth, the particles would be thrown beyond the control of the sun and would be dissipated into space.

We are compelled to acknowledge that up to the present time science has rendered no satisfactory account of the origin of the solar light or heat. Whence comes the exhaustless supply, scattered so lavishly into space in every direction, we know not. Neither is it possible to give a

satisfactory solution of the solar spots, or of any of the strange phenomena attending their rotation or translation on the sun's surface. The idea that tornadoes and tempests rage in the deep, luminous ocean that surrounds the sun, like those which sometimes agitate the atmosphere of the earth, has no solid foundation. We know the exciting causes of the tornadoes on earth, but why such storms should exist in the solar photosphere it is in vain to conjecture at present. Doubtless the time will come when these phenomena will be explained. Persevering and well-directed observation will, in the end, triumph; but these are matters which must be consigned to the researches of posterity.

CHAPTER II.

MERCURY, THE FIRST PLANET IN THE ORDER OF DISTANCE FROM THE SUN.

Its Early Discovery.-Difficult to be distinguished from the Stars.Elongations.-Motion Direct and Retrograde.-Sometimes Stationary. -Nature of the Orbit.-Variation in the Elongation explained.-The Nodes.-Transit of Mercury.-Inclination of Mercury's Orbit.-Mean Distance from the Sun.-Conjunctions. - Phases. - Diameter and Volume.

No discovery made by the ancients gives us a higher idea of the care and scrutiny with which their astronomical observations were conducted than the fact that the minute planet Mercury, so difficult to be seen, and so undistinguishable from the fixed stars, was discovered in the very earliest ages of the world. That the brighter planets, such as Venus and Jupiter, whose brilliancy exceeds that of any of the fixed stars, should have been detected to be wandering bodies, even in the remotest antiquity, is by no means surprising. For in watching the sun rising and the sun setting, so as to note, in the first instance, the stars nearest to the sun, which were the last to fade away, and in the second, those stars which

were the first to become visible, the change of position of the planets Venus and Jupiter could not fail to attract the attention of the student of the heavens; but the planet Mercury is so small, and so rarely visible even to the keenest eye, that it is said Copernicus himself, during his whole life devoted to the study of the heavens, never once caught sight of this almost invisible world.

Mercury, in his appearance to the naked eye, is not distinguishable from the fixed stars. His close proximity to the sun, the fact that he is never visible except near the horizon, and the intense brilliancy of his disc give to him that twinkling appearance which distinguishes the fixed stars. Notwithstanding all these difficulties, the oldest astronomers managed to acquire a very complete knowledge of the principal facts connected with the movement of this planet. By a careful and continuous examination it was found that Mercury never receded more than about twentyeight degrees from the sun's centre. The amount of recess, or elongation as it is called, was soon discovered to be a variable quantity, a fact which demonstrated that in case the planet revolved in a circular orbit, inclosing the sun, the sun could not occupy the centre of this circle. By watching the elongations from revolution to revolution, it was found that they varied from a minimum of 16° 12', to a maximum of 28° 48'. Knowing the amount of this variation, and watching carefully the progressive change, it became possible to reach a tolerably accurate knowledge of the nature of the orbit described by the planet in its revolution around the sun. It was soon discovered that in some portions of his orbit Mercury advanced with the sun in his march among the fixed stars, while in other parts of his orbit his motion became retrograde, and in the change from direct to retrograde, and the reverse, the planet apparently ceased to move, and for a short time became stationary.

It will be seen that all these changes are readily accounted for by supposing the planet to revolve about the sun in a circular orbit, the sun being eccentrically placed.

If we conceive two visual rays to be drawn from the eye of the observer, and tangent to the orbit of Mercury on the right and on the left, the planet, while traversing that arc of its orbit intercepted between the points of contact and nearest to the eye, will move direct; in passing through the point of contact after direct motion ceases, it will move off in the direction of the visual ray, and hence will appear stationary for a short time. In the larger portion of its orbit (that remote from the eye) its motion must be opposite to that of the sun, and hence retrograde. In coming up to the second point of contact, the planet will move along the visual ray toward the eye of the observer, and hence for a short time will appear stationary.

To account for the variation in the elongations of Mercury, we must either suppose the point of nearest approach of the planet to the sun, called its perihelion, to be in motion, or else we must suppose the spectator to be himself moving, and thus to behold the planet, its perihelion point, and the sun, under varying relations to each other. As the early astronomers assumed the immobility of the earth, they explained the variations in the elongations of Mercury by giving to its perihelion point a motion of revolution about the sun.

It is impossible to follow the planet with the naked eye in its close approach to the solar orb, as its feeble reflected light is necessarily overpowered by the brilliancy of the sun, but by close observation, and by marking the positions of the planet at its disappearance and reappearance, the old astronomers are said to have reached to a knowledge of the fact that this planet sometimes crosses the sun's disc, producing what is called a transit of Mercury, identical in its phenomena with the transit of Venus, already spoken of in connection with the determination of the solar parallax. In case the plane of the orbit of Mercury were exactly coincident with the plane of the sun's apparent orbit, it is manifest that every revolution of the planet would produce a transit.

As this, however, is not the case, and as no central