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THE BEAUTY OF THE HEAVENS.

planet, with its satellites in their respective orbits, is shewn, illuminated by the sun,from the right-hand upper corner of the picture. In the lower corner, on thesame side, is seen the earth ; lines drawn from the body of the planet, andfrom each satellite to the earth, shew the directions in which an observer seesthem. The planet’s shadow is thrown, as in nature, in the direction opposite tothe sun. The first, or nearest satellite, is seen entering the shadow ; to theobserver on the earth, it is eclipsed ; the second satellite is passing behind thebody of the planet, and is, therefore, invisible from the earth ; the third hasjust emerged from the planet’s shadow, and is seen on its left; the fourthis seen at a greater distance from the planet, and on the right of it. Whenthe satellites pass through that portion of their orbits nearest the sun, wesometimes see their shadows passing over the body of the planet. The appli-cation of our knowledge of these satellites and their motions, to important pur-poses of science, has been already mentioned, when describing the planet itself.

Scene No. LXX. —Comets.

A general account of the nature of comets has been already given. Thenumber of these wanderers, that have been seen within the limits of oursystem,is variously stated at from 350 to 500. The probable number existing, in theopinion of the most scientific men of the age, amounts to more than sevenmillions ! The orbits of about 100 comets have been determined ; of thesethree have known periods of return, and may, therefore, be said to formpart of our system. The first is called Halley’s comet, of which the periodof revolution is about seventy-eight years ; the second, Encke’s comet, whoseperiod of revolution is about three years and four months ; the third is thatcalled Biela’s comet, whose revolution is made in about six years and threequarters. This is the comet which it was seriously predicted would, in 1832,its period of return, come into collision with the earth.

The scene gives representations of two comets of the most remarkableappearance: the upper figure is the great comet of 1680 ; it passed round thesun at a distance of only 147,000 miles, moving at the rate of 880,000miles an hour. At that time the tail of the comet, of the form shewn in thescene, was sixty millions of miles in length, and became gradually extended tillit attained a length of 120 millions of miles. On its near approach to the sun,the sun’s body, if viewed from the comet, would have nearly covered the wholeextent of the heavens from the horizon to the zenith ; and as this comet is pre-sumed to have a period of 575 years, the sun would, at its remotest distance,appear little larger than one of the fixed stars to us. As the tail of this cometextended over ninety degrees of space, when its head had set below the horizon,the tail still reached the zenith.

The lower figure of the scene is a representation of the. remarkable cometof 1811. The tail of this comet consisted of two diverging beams of light,