LET. III. FIRMAMENT OF THE FIXED STARS. 39

he performs his diurnal rotation in 9 hours and56 minutes.

Saturn, the next planet in the system aboveJupiter, is about 900 millions of miles fromthe Sun; and by travelling at the rate of twenty-two thousand miles an hour, he performs hisannual circuit round that luminary in about29 j of our years. His diameter is computedto be feventy-nine thousand miles ; but, on ac-count of his immense distance, and the defici-ency of light occasioned by such a remote situa-tion, the time of his diurnal rotation upon hisaxis has not yet been discovered.

This has commonly been thought the mostremote planet in our system; but another, at astill greater distance, called the GeorgiumSidus,has lately been discovered by Mr. Herschel, theparticulars of which, as they are new and in-teresting, I shall speak of more at large in somefuture letter. At present it is only necessaryto observe, that besides the primary planets herementioned, there are ten others, called secon-dary planets, or satellites, which regard theirprimaries as the centers of their motions, andrevolve round them in the fame manner asthose primaries revolve round the Sun.

One of the most conspicuous of these satellitesis the Moon, who is a constant attendant on ourEarth; and, whilst she accompanies it in itsannual progress through the heavens, keepsD 4 revolving