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 satel lites, 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 of 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 about 12 millions of miles in a minute.

James. Who discovered these satellites?

Tutor. They were first seen by Galileo in 1610. He took them for telescopic stars, but farther observations convinced him and others that they were planetary bodies.

The relative situation of these small bodies changes at every instant. They are sometimes seen to pass over the face of the planet, and project a shadow in the form of a black spot, which describes a line across it.

CONVERSATION XXII.

Of Saturn.

TUTOR. We are now arrived at Saturn in our descriptions, which, till within these forty 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 thousand miles in length: his distance from the sun is more than 900 million of miles, and he performs his

journey round that luminary in a little less than 30 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 expérience here.

Tutor. His distance from the sun being between 9 and 10 times greater than that of the earth, he enjoys 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 that which 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 even here was 500