Mercury is the planet nearest the sun; his orbit is consequently contained within ours; his vicinity to the sun, prevents our frequently seeing him, so that very accurate observations cannot be made upon Mercury. He performs his revolution round the sun in about 87 days, which is consequently the length of his year. The time of his rotation on his axis is not known; his distance from the sun is computed to be 37 millions of miles, and his diameter 3180 miles. The heat of this planet is supposed to be so great, that water cannot exist there but in a state of vapour, and that even quicksilver would be made to boil. _Caroline._ Oh, what a dreadful climate! _Mrs. B._ Though we could not live there, it may be perfectly adapted to other beings, destined to inhabit it; or he who created it may have so modified the heat, by provisions of which we are ignorant, as to make it habitable even by ourselves. Venus, the next in the order of planets, is 68 millions of miles from the sun: she revolves about her axis in 23 hours and 21 minutes, and goes round the sun in 244 days, 17 hours. The orbit of Venus is also within ours; during nearly one-half of her course in it, we see her before sun-rise, and she is then called the morning star; in the other part of her orbit she rises later than the sun. _Caroline._ In that case we cannot see her, for she must rise in the day time? _Mrs. B._ True; but when she rises later than the sun, she also sets later; so that we perceive her approaching the horizon after sun-set: she is then called Hesperus, or the evening star. Do you recollect those beautiful lines of Milton? Now came still evening on, and twilight gray Had in her sober livery all things clad; Silence accompanied; for beast and bird, They to their grassy couch, these to their nests Were slunk, all but the wakeful nightingale; She all night long her amorous descant sung; Silence was pleas'd; now glowed the firmament With living sapphires. Hesperus that led The starry host, rode brightest, till the moon Rising in clouded majesty, at length Apparent queen unveil'd her peerless light, And o'er the dark her silver mantle threw. The planet next to Venus is the Earth, of which we shall soon speak at full length. At present I shall only observe that we are 95 millions of miles distant from the sun, that we perform our annual revolution in 365 days 5 hours and 49 minutes; and are attended in our course by a single moon.