CONVERSATION XLII.

Of Jupiter.

Tutor. We now come to Jupiter, the largest of all the planets, which is easily known by his peculiar magnitude and brilliancy.

Charles. Is Jupiter larger than Venus?

Tutor. Though he does not appear so large, yet the magnitude of Venus bears but a very small proportion to that of Jupiter, whose diameter is 90,000 miles in length, consequently his bulk will exceed the bulk of Venus 1500 times: his distance from the sun is estimated at more than 490 millions of miles.

James. Then he is five times farther from the sun, than the earth, and consequently, as light and heat diminish in the same proportion as the squares of the distances from the illuminating body increase, the inhabitants of Jupiter enjoy but a twenty-fifth part of the light and heat of the sun that we enjoy.

Tutor. Another thing remarkable in this planet is, that it revolves on its axis, which is perpendicular to its orbit, in 10 hours, and in consequence of this swift diurnal rotation, his equatorial diameter is 6000 miles greater than his polar diameter.

Charles. Since then a variety in the seasons of a planet depends upon the inclination of the axis to its orbit, and since the axis of Jupiter has no inclination, there can be no difference in his seasons, nor any in the length of his days and nights.

Tutor. You are right, his days and nights are always five hours each in length; and at his equator, and its neighbourhood, there is perpetual summer; and an everlasting winter in the polar regions.

James. What is the length of his year ?

Tutor. It is equal to nearly 12 of ours, for he takes 11 years, 314 days, and 10 hours, to make a revolution round the sun; consequently he travels at the rate of more than 28,000 miles in an hour.

This noble planet is accompained with four satellites, which revolve about him at different distances, and in different periodical times; the first in about 1 day and 11 hours: the second in 3 days, 13 hours: the third in 7 days, 3 hours; and the fourth in 16 days and 16 hours.

Beyond the sphere of Mars in distant skies,
Revolves the mighty magnitude of Jove
With kingly state, the rival of the sun.
About him round four planetary moons
On earth with wonder all night long beheld
Moon above moon, his fair attendants dance.

MALLET'S EXCURSION.

Charles. And are these satellites, like our

moon, subject to be eclipsed?

Tutor. They are; and their eclipses are of considerable importance to astronomers, in ascertaining with accuracy the longitude of different places on the earth.

By means of the eclipses of Jupiter's satellites, a method has been also obtained of demonstrating that the motion of light is progressive, and not instantaneous, as was once supposed. Hence it is found that the velocity of light is nearly 11,000 times greater than the velocity of the earth in its orbit, and more than a million of times greater than that of a ball issuing from a cannon. This discovery is alluded to by the last-mentioned poet: speaking of an observer of the eclipses and Jupiter's satellites, he says:

By these observ'd the rapid progress finds
Of light itself; how swift the headlong ray
Shoots from the sun's height through unbounded space,
At once enlight'ning air, and earth, and heaven.

Rays of light come from the sun to the earth in 8 minutes, that is, at the rate of 12 millions of miles in a minute nearly.

U 2

CONVERSATION XLIII.

Of Saturn.

Tutor. We are now arrived at Saturn in our descriptions, which, till within these twenty years, was esteemed the most remote planet of the solar system.

Charles. How is he distinguished in the heavens ?

Tutor. He shines with a pale dead light, very unlike the brilliant Jupiter, yet his magnitude seems to vie with that of Jupiter himself. The diameter of Saturn is nearly 80,000 miles in length his distance from the sun is more than 900 millions of miles, and he performs his journey round that luminary in a little less than 50 of our years, consequently he must travel at a rate not much short of 21,000 miles an hour.

James. His great distance from the sun must render an abode on Saturn extremely cold and dark too, in comparison of what we experience here.

Tutor. His distance from the sun being between 9 and 10 times greater than that of the earth, he must enjoy about 90 times less light and heat; it has nevertheless been calculated that the light of the sun at Saturn is 500 times greater than we enjoy from our full moon.

Charles. The day-light at Saturn, then, cannot be very contemptible: I should hardly have thought that the light of the sun here was 500 times greater than that experienced from a full

moon.

Tutor. So much greater is our meridian light than this, that during the sun's absence behind a cloud, when the light is much less strong than when we behold him in all his glorious splendour, it is reckoned that our day-light is 90,000 times greater than the light of the moon at its full. James. But Saturn has several moons I believe?

Tutor. He is attended by seven satellites, or moons, whose periodical times differ very much; the one nearest to him performs a revolution round the primary planet in 22 hours and a half; and that which is most remote takes 79 days and 7 hours for his monthly journey: this last satellite is known to turn on its axis, and in its rotation is subject to the same law which our moon obeys, that is, it revolves on its axis in the same time in which it revolves about the planet. Besides the seven moons, Saturn is encompassed with two broad rings which are probably of considerable importance in reflecting the light of the sun to that planet; the breadth of the inner ring is 20,000 miles, that of the outer ring 7200 miles, and the vacant space between the two rings is 2839 miles. These rings give Saturn a very different appearance to any of the other