tion. The theory subsequently received a great extension from the ingenious labors of Fresnel; and the still more recent researches of Arago, Poisson, Herschel, Airy, and others, have conferred on it so great a degree of probability, that it may almost be regarded as ranking in the class of demonstrated truths. "It is a theory," says Herschel, "which, if not founded in nature, is certainly one of the happiest fictions that the genius of man has yet invented to group together natural phenomena, as well as the most fortunate in the support it has received from whole classes of new phenomena, which at their discovery seemed in irreconcilable opposition to it. It is, in fact, in all its applications and details, one succession of felicities; inasmuch as that we may almost be induced to say, if it be not true, it deserves to be."

Light and heat are so intimately related to each other, that philosophers have doubted whether they are identical principles, or merely coexistent in the luminous rays. They possess numerous properties in common: being reflected, refracted, and polarized, according to the same optical laws, and even exhibit the same phenomena of interference. Most substances during combustion give out both light and heat; and all bodies, except the gases, when heated to a high temperature, become incandescent. Nevertheless, there are many circumstances in which they appear to differ.

A thin plate of transparent glass interposed between the face and a blazing fire intercepts no sensible portion of the light, but most sensibly diminishes the heat. Light and heat are therefore not intercepted alike by the same substances. Heat is also combined in different degrees with the different rays of the solar spectrum. A very remarkable discovery on this subject was made by Sir William Herschel, which would seem to establish the independence of the heating and illuminating effects of the solar rays. Having placed thermometers in the several prismatic colors of the solar spectrum, he found the heating power of the rays gradually increased from the violet (where it was least) to the extreme red, and that the maximum temperature existed some distance beyond the red, out of the visible part of the spectrum. The experiment was soon after repeated with great care by Berard, who confirmed Herschel's conclusions relative to the augmentation of the calorific power from the violet to the red, and not beyond the spectrum. This discovery of the inequality of the heating power of the different rays led to the inquiry whether the chemical action produced by light upon certain bodies was merely the effect of the heat accompanying it, or owing to some other cause. By a series of delicate experiments, Berard found that this action is not only independent of the heating power, but follows entirely a different law; its intensity being greater in the violet ray, where the heating power is the least, and least in the red ray, where the heating power is the greatest. We are thus led to the conclusion that the solar rays possess at least three distinct powers-those of heating, illuminating, and effecting chemical combinations and decompositions; and these pow ers are distributed among the different refrangible rays in such a manner as to show their complete independence of each other.

I shall dismiss this subject, however, for the present, as I shall have another opportunity of more fully developing the relations of heat and light.

THE MAJOR PLANETS.

Space between MARS and JUPITER.-Jupiter's Distance and Period.-His Magnitude and Weight.— His Velocity.-Appearance of his Disk.-Day and Night on Jupiter.-Position of his Axis.—Absence of Seasons.-His Telescopic Appearance.-His Belts.-Causes of his Belts.-Currents in his Atmosphere.-Madler's Telescopic Views of Jupiter.-Appearance of the Sun as seen from Jupiter. His Satellites.-The Variety of his Months.-Magnificent Appearance of the Moons as seen from Jupiter.-Their Eclipses.-SATURN.-His diurnal Rotation.-Appearance of the Sun as seen from him.-His Atmosphere.-His Rings.-Their Dimensions.-Biot's Explanation of their Stability-Herschel's Theory of the same.-Appearances and Disappearances of the Rings-Various Phases of the Rings.- Saturn's Satellites.-HERSCHEL or URANUS.-His Distance and Magnitude.-His Moons.-Reasons why there is no Planet beyond his Orbit.

THE MAJOR PLANETS.

PASSING across the wide space which intervenes between the minor planets which, with the earth, circulate under the immediate wing of the sun, in the midst of which space we encounter the strange spectacle of the ruins of a shattered world, we arrive at the region of the system in which roll in silent majesty the stupendous orbs of JUPITER, SATURN, and HERSCHEL, accompanied by their gorgeous apparatus of multiplied moons, rings, and belts. The mind is prepared to expect here another order of worlds, and it is not disappointed. The first of these sublime globes which attracts our attention is that of JUPITER, whose diameter is eighty-eight thousand miles, and whose bulk is fifteen hundred times that of our own globe. The distance of this planet from the sun is nearly five hundred millions of miles, and when our globe is nearest to it, it is nearly four times more distant from us than the sun. Nevertheless, such is its stupendous size that it subtends to the eye an angle of forty-five seconds, and is, next to the sun and moon, the most brilliant object in the heavens. It has in this respect the advantage over VENUS, that when nearest to us its illuminated hemisphere is presented directly to the line of vision, and it is seen in the meridian at midnight, when the entire absence of the sun's light so much favors its apparent splendor. The orbit of the earth, which is included in that of Jupiter, is so small, compared with that of the planet, that its illuminated hemisphere, which is presented precisely to the sun, is always presented very nearly to the earth. Jupiter, therefore, does not appear sensibly gibbous, and, consequently, is always seen with a full face.

The time which Jupiter takes to make his complete revolution round the sun, is 4,333 days, being something less than twelve years. Such is the length of the year of Jupiter.

The weight or mass of the planet Jupiter is 316 times greater than that of the earth; but its bulk, being greater than that of the earth, in the higher proportion of about fifteen hundred to one, it follows that its density is about four times less than that of the earth; being nearly equal to the density of the sun.

The globe of Jupiter is therefore about as heavy as if it was composed of

water from its surface to its centre.

There is nothing connected with the motion of the planets more surprising than their enormous velocities, which, to our observation, are nevertheless scarcely perceptible, owing to the fact that their distances from us are proportionally great. Jupiter, when nearest to us, is at a distance of four hundred millions of miles. A cannon-ball which moves at the rate of five hundred miles an hour, would require nearly a hundred years to come from Jupiter to us, and if a steam-engine on a railway, moving at twenty miles an hour, were to take its departure for Jupiter, it would not arrive at its destination until the expiration of two thousand three hundred years.

Taking the diameter of Jupiter's orbit at a thousand millions of miles, its circumference is more than three thousand millions of miles, which is traversed in less than twelve years. The space moved over annually by Jupiter is, then, two hundred and fifty millions of miles; and the space moved over monthly about twenty millions of miles; and the space moved over daily about seven hundred thousand miles; and the space moved over hourly about thirty thousand miles; being at the rate of about five hundred miles a minute; a velocity sixty times greater than that of a cannon-ball.

DIURNAL ROTATION OF JUPITER.

Although the varieties of light and shade which characterize the disk of Jupiter are subject to variations which show, as will be seen hereafter, that they are principally produced by clouds in his atmosphere, yet permanent marks were discovered upon it at an early epoch, by which the fact was established that the planet has a diurnal rotation. In the years 1664-'5, Hook and Cassini observed a spot on one of the belts which was permanent in its position, and was observed to move across the disk of the planet. It contracted in its breadth as it approached the edge of the disk; a circumstance which obviously arose from its being fore-shortened by the position in which it was there presented to the eye, that portion of the surface of the planet being seen very obliquely, the spot disappeared at one side, and after being invisible for a time reappeared at the other. This spot continued to be seen for more than a year, and fully proved the fact that Jupiter completes his rotation on an axis very slightly inclined to his orbit in nine hours and fifty-six minutes.

The alternations of light and darkness on Jupiter are therefore regulated by intervals much shorter than those which govern the days and nights of the minor planets, and we shall presently see that this is a character which probably prevails among all the major planets. The average interval of the days and nights must be a little under five terrestrial hours.

This rapid motion, considered with reference to the great magnitude of Jupiter, leads to the inference that the velocity of that part of his surface which is near his equator must be exceedingly great. The circumference of Jupiter at his equator must be about two hundred and seventy thousand miles, and as this revolves in ten hours, the motion of any point upon it must be at the enormous rate of twenty-seven thousand miles an hour, or a little less than five hundred miles a minute. Thus it appears that the velocity which the equatorial regions have, in virtue of the diurnal motion, is very little less than the orbitual motion of the planet round the sun.

This rapid diurnal rotation would produce a considerable variation in the weights of bodies at different latitudes on the surface of Jupiter, since the centrifugal force near the equator would counteract the weight in a very sensible manner, while toward the poles its effects would cease to be perceptible.