OF THE SOT AR SYSTEM, AND THE

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which, by means of a telescope, are constantlyto be seen on their bodies, we are able to de-monstrate that they must have such a motion,round their axes, as answers to the diurnalrotation of the earth. And from their seemingat some times to be stationary, and at othertimes to- go backwards, or be retrograde, weare also equally certain, that they must have sucha progressive motion round the sun, as answersto the annual revolution of the earth in itsorbit.

Mercury, the nearest planet to the fun, goearound him in about cighty-seven days andtwenty-three hours, or a little less than threemonths; which is the length of his year. Butbeing seldom seen, on account of his nearnessto the fun, and no ipots appearing on his sur-face, or dilk, the time of his rotation upon his-axis, or the length of his days and nights, is asyet unknown. His distance from the fun iscomputed to be about thirty-feven millions ofmiles, and his diameter three thousand twohundred; and in his course round the sun,he moves at the rate of a hundred and fivethousand miles an hour.

This planet, when viewed, in different posi-tions, with a good telescope, seems to have allthe phases or appearances of the moon, exceptthat he can, at no time, be seen entirely round,or quite full ; because his enlightened side is

never