We are now brought to the planet Jupiter, by far the largest, and, in many respects, the most remarkable of all the planets. When a telescope is directed to Jupiter, the observer is first struck by the appearance of several much smaller bodies, evidently accompanying the planet in his orbit about the sun, and revolving about him in the same manner as the the moon revolves about the earth. These are the four satellites of Jupiter, affording upon a small scale an epitome of the whole solar system. These satellites all revolve from west to east about their primary planet, in planes which are inclined at small angles to the orbit of Jupiter.

Their mean distances, expressed in equatoreal radii of the planet, their periodic times, expressed in mean solar days, and their diameters in miles, are as follows:

It will be found, that the squares of the periodic times of these satellites are to one another, as the cubes of the mean distances; according to Kepler's third law. The diameter of the moon being 2160 miles, it will be seen, that three of the satellites are considerably larger than the moon.

The first three satellites of Jupiter are totally eclipsed in every revolution; and the fourth satellite is also frequently eclipsed. If Jupiter is near the sun, these eclipses are invisible at the earth; and again when Jupiter is exactly in opposition to the sun, or in the line joining the centres of the sun and the earth, the body of Jupiter ap pears larger, as seen from the earth, than the section of his shadow at the place where a satellite passes through it. Hence any satellite at the instant of eclipse is hidden from the view of a spectator on the earth, behind, the body of the planet; and the eclipse is consequently invisible.

In all other positions of Jupiter, either the ingress or egress of the satellite at the time of an eclipse will be visible from those points of the earth in which Jupiter is above the horizon at night and these

eclipses, previously calculated, serve, as we have already seen in a previous number, to determine the longitude of places upon the earth's surface.

The satellites are sometimes observed to pass across the disk of Jupiter, projecting visible shadows upon the surface of the planet, and thus causing a total eclipse of the sun upon those parts of Jupiter over which they pass. They are also occasionally hidden from view by passing behind the planet. Hence, it frequently happens, that not more than two of the satellites are visible: sometimes only one; and very rarely, Jupiter is seen, for a short time, as if unaccompanied by any satellite.

The position of Jupiter's satellites, as seen at a certain hour of every night in which the satellites are visible, is given in the Nautical Almanac.

The following eclipses of Jupiter's satellites will be visible at Greenwich, and consequently in almost all parts of Great Britain, in the month of September, 1844: the column containing the words Im. or Em, implies that the immersion of the planet into the shadow, or the Emersion from the shadow, is visible.

ECLIPSES OF JUPITER'S SATELLITES.

The surface of Jupiter itself is observed to be crossed by variable dark bands, or belts, in a direction nearly parallel to the direction of Jupiter's equator, and to that in which his satellites appear. These belts seem to be occasioned by the atmosphere of Jupiter, as affected by difference of climate, in the same manner as the earth is influenced by the position of the several parts of her surface with respect to the

sun.

By observations upon some more prominent spots upon the planet, it is found, that Jupiter revolves about an axis perpendicular to the direction in which the belts lie, in 9h. 55m. 50s, sidereal time. Since the equatoreal diameter of the earth, is about 7925 miles, and that of Jupiter about 87000 miles, which are to one another nearly in the ratio of 10 to 109, and the earth revolves in 24 sidereal hours, or 86,400 seconds, while Jupiter revolves in 35,750 seconds, these num. bers being to one another nearly in the ratio of 24 to 10; it follows, that a point on Jupiter's equator moves about its axis with a velocity greater than that of a point on the earth's equator about its axis, in the proportion of the product of 109 and 24 to 100, or nearly as 26 to 1.

This great rapidity of rotatory motion accounts for the evidently oblate form of Jupiter, which is such as at once to strike the eye by its orange-like shape, the proportion of the equatoreal and polar diameters being found by measurement to be nearly that of 107 to 100. This is precisely the proportion which arises from computation of the form of equilibrium which a body would assume from the mutual gravitation of its parts, when revolving with the same angular velocity as Jupiter, thus furnishing a remarkable proof of the accuracy of the principles upon which those calculations are founded.

The next planet in our system, Saturn, is nearly as large as Jupiter, his diameter being 79,060 miles, or nearly ten times as great as that of the earth. His magnitude is consequently 1000 times greater than that of the earth; but his mass is little more than 100 times greater than the earth. Hence his density is only about one tenth of the density of the earth, or about one-half of the density of water. His surface is marked with belts somewhat similar to those of Jupiter. The form of Saturn is remarkable; it is not unlike that of a loaf of bread, and its outline somewhat resembles that which would be made by a square, rounded off at the angles. This peculiarity of Saturn's form is found to arise from the attraction of his rings, which form a most wonderful appendage to the planet.

Saturn is surrounded by two broad and flat rings, extremely thin and lying in one plane, which is slightly inclined to the planet's equa tor. The two rings are separated throughout their whole extent by a narrow interval. It may lead us to form some conception of the magnitude of these rings to state, that if their centre coincided with that of the earth, the distance from the surface of the earth to the inner ring would be about 43,000 miles; and the distance from the surface

of the earth to the extremity of the outer ring about 84,000 miles; the diameter of the outer ring being more than one-third of the diameter of the moon's orbit.

The following are the dimensions of the planet and its rings, as computed by Sir John Herschel, from Professor Struve's and his own observations.

Interval between the planet and the interior ring 19,090.
Intervals of the rings

Thickness of the rings not exceeding..

1,791.
100.

When we consider the planet Saturn and his rings, as sustained in permanent equilibrium by their mutual attraction, and by the forces originally impressed upon them, we are lost in astonishment. Conceive a ball launched in space, surrounded by two such rings, independent of one another, all acted on by the attraction of the sun and of the planets, as well as by one another and by the satellites of the planet, and that the stability of the system depends upon the motions being so adjusted, that none of these bodies shall be separated from each other, that the rings shall neverbe precipitated upon either one another, or upon the surface of the planet: and then consider, that the complicated motions of these masses are carried on from age to age with perfect regularity; and we shall form some conception of the wisdom and power thus displayed.

The physical cause, upon which the possibility of such conditions being fulfilled depends, is that the rings should not be of uniform density. Had they been so, the equilibrium of the system of rings might have subsisted for an instant; but it would have been of the kind which is called unstable equilibrium ; like that of a needle balanced on its point. The slightest disturbance, such as must have arisen from the action of the planets, would have given rise to a series of changes which would eventually have brought the planet and its rings into permanent contact. As the system is adjusted, the rings revolve about the planet, in 10h. 29m. 17s., which agrees very nearly with the periodic time of a satellite revolving at a distance equal to the mean distance of the rings from Saturn and thus, where irregularity of form is requisite to produce permanence, irregularity is provided,

In the present month, (September 1844) the northern portion of the

ring is visible. Its appearance is that of an ellipse of which the major axis (or longest line across) is about 40", and the minor axis, (or shortest line across) about 14". This is nearly the most favourable time in the present year for observing the rings.

There are certain periods, occuring at intervals of about 15 years, when the plane of the rings, which cuts the ecliptic in two points situated in 170° and 350° of longitude, passes through the sun. At such times, the edge only of the rings is illuminated, forming a line of light of such extreme thinness, as to be visible only under the most favourable circumstances and with the most powerful telescopes. The next disappearance of the rings will take place in 1853.

The rings cast a distinct shadow upon the body of the planet, and must occasion a total solar eclipse, in some parts of his surface, of nearly 15 years duration.

At other periods, the appearance of the ring, seen from the planet, as a stupendous luminous arch, spanning the whole sky, probably diversified with spots sufficiently distinct to measure time by the rapid movement of the ring, and eclipsing the satellites in their revolution, must afford a magnificent spectacle.

The satellites of Saturn are seven in number. The two farthest from the planet are the largest; the two inner ones are extremely small, so as to be invisible except by the most perfect telescopes. From observations upon changes of brightness in the largest satellite of Saturn, and in those of Jupiter, it has been concluded, that these satellites, like the moon, all revolve about their axis in the same time in which they revolve in their orbit, thus always presenting the same side to their respective primary planets.

The planet Uranus was discovered by Sir William Herschel, on the 13th of March, 1781. Notwithstanding the great real magnitude of this planet, its diameter being about 35,000 miles, more than four times that of the earth, its magnitude 64 times that of the earth, and its mass about 20 times that of the earth, it appears to the naked eye as a scarcely visible star, and, when viewed through a telescope, subtends an angle of only 4". This planet has no sensible rings or belts. He is surrounded by several satellites, two of which have been carefully observed, and four others are supposed to exist.

It is very remarkable, that these satellites of Uranus differ in two respects from all the other heavenly bodies in the solar system. The planes of their orbits, instead of coinciding nearly with that of the ecliptic, are nearly perpendicular to that plane: and the directions of