A ring of matter.

hypothetical explosion must have occurred hundreds of thousands or millions of years ago, if ever.

According to the nebular hypothesis (to be set forth hereafter), the asteroids may have arisen from the condensation of a ring of nebulous matter, which was left behind, as the original solar nebula contracted. This is the commonly received explanation of their origin.

CHAPTER XV.

JUPITER, SATURN, URANUS, AND NEPTUNE.

"Some displaying

Enormous liquid plains, and some begirt

With luminous belts, and floating moons, which took,
Like them, the features of fair earth."

-Byron.

JUPITER is the giant of the sun's family of planets.

He is

He

Jupiter's

and shape.

The distance from pole to pole is over 84,000 miles. At dimensions the equator his diameter is nearly 90,000 miles. therefore decidedly out of round. The elliptical shape of his disc is readily perceived with a telescope, or in any good picture of him. So marked an equatorial bulge may be due to one or both of two causes. may rotate with extreme rapidity, so that the "centrifugal force" at the equator is large, or he may plastic that even a low velocity of rotation would cause the bulging observed. As we shall see presently, there is good reason to believe that both of these causes operate.

be so

revolution.

His bulk is 1,300 times that of the earth; all the Size, distance, other planets compacted together into one would not and time of equal him in volume. His mean distance from the sun is 483,000,000 miles, which is more than five times the earth's; 11.86 years are occupied in one revolution about the sun.

Like all other superior planets he is brightest at His appearopposition, attaining then a luster which exceeds that of ance. any other planet except Venus; at such a time he casts

Rotation.

perceptible shadows of terrestrial objects. Many spots can be seen on his surface, even with a telescope of moderate power; by watching their motion the time of rotation has been determined; it is about 9hrs. 55min.

The belts.

The swiftness of rotation makes the delineations of its surface markings difficult.

In a small telescope dark belts parallel to the planet's equator are plainly contrasted with the general yellowishwhite background. A large telescope reveals a wealth of detail and a richness of coloring, which call forth the admiration of the beholder. The principal belts near the equator have a reddish cast; the hue is modified

from time to time, being sometimes salmon-colored and at others a rich rose pink. There are many subsidiary stripes of smaller size and less pronounced color.

The whitish portions of the planet's disc are by no means devoid of interest. They look like aggregations of cumulus clouds, such as deck the summer sky. One who looks down from the top of a mountain upon a layer of clouds below may see the general aspect of the Jovian clouds. Small white, dark, and red spots are

strewn here and there over the surface.

White clouds.

The great

In 1878 there suddenly appeared a pink spot of unprecedented dimensions; the length is given as 30,000 red spot. miles, the breadth as 7,000. In another year its hue

was a full Indian red. So completely did it dwarf all other recorded spots that it was henceforth known as the "great red spot." It faded away, and was almost invisible in 1883 and 1884. Since then it has had irregular spells of brightening, but has never recovered its pristine beauty. The time of rotation of the red spot is not the same as that of the adjacent cloudforms. In 1890 a large spot was moving directly toward the red spot; but it was diverted from its course, and passed by at one side of the spot. After it passed by it did not return to its original course, but remained at the higher latitude into which it had been shunted; it passed the red spot at the rate of twenty miles an hour. Professor Keeler has likened the great red spot to a sand bank in a river, past which the flecks of foam go scurrying.

The red belts are thought to be cloudless regions; The red belts. the sunlight striking against the whitish cloud-masses is reflected back in large measure; but that which falls upon the red rifts between the clouds is not so well re

* Prof. James E. Keeler, of the Allegheny Observatory.

A red atmosphere.

flected. If Jupiter's atmosphere is red and the white masses are clouds floating in it at various heights, the general appearances are explained. What we have called the atmosphere may be a liquid having a reddish color. Not only do the different parts of Jupiter's cloud mantle rotate with different velocities, but even the

Variable rotation.

great red spot has not kept a constant period of rotation. At first the whirling of the planet on its axis brought it around in 9hrs. 55min. 34sec. In seven years the period had lengthened seven seconds. If it had kept the new rate and Jupiter itself had been a solid rotating at the old rate, it would have gone clear around