THE PLANETS. 131 



ets, its constitution, and height. I will merely call to mind 

 here the conjecture of Sir John Herschel, as to the temper- 

 ature of the Moon's surface, " which must necessarily be very 

 much heated possibly to a degree much exceeding that of 

 boiling water."* 



b. SECONDARY PLANETS. 



The general comparative considerations relating to the 

 secondary planets have already been given with some com- 

 pleteness in the delineations of nature (Cosmos, vol. i., p. 

 94-98). At that time (March, 1845) there were only 11 

 principal and 18 secondary planets known. Of the asteroids 

 so called telescopic, or small planets, only four were discov 

 ered : Ceres, Pallas, Juno, and Vesta. At the present time 

 (August, 1851), the number of the principal planets exceeds 

 that of the satellites. We are acquainted with 22 of the for- 

 mer and 21 of the latter. After an intermission of thirty- 

 eight years in planetary discoveries (from 1807, to December, 

 1845), commenced a long series of ten new small planets, 

 with Astrea, discovered by Hencke. Of these, two (Astrea 

 and Hebe) were first detected by Hencke at Driesen, four 

 (Iris, Flora, Victoria, and Irene) by Hind in London, one (Me- 

 tis) by Graham at Markree Castle, and three (Hygeia, Par- 

 thenope, and Egeria) by De Gasparis at Naples. The dis- 

 covery of the outermost of all the large planets, Neptune, an- 

 nounced by Leverrier, and found by Galle at Berlin, followed 

 ten months after Astrea. The discoveries now accumulate 

 with such rapidity, that the topography of the solar regions 

 appears, after the lapse of a few years, quite as antiquated as 

 statistical descriptions of countries. 



Of the 21 satellites now known, one belongs to the Earth, 

 four to Jupiter, eight to Saturn (the last discovered of these 

 eight is, according to distance, the seventh, Hyperion ; discov- 

 ered in two different places at the same time by Bond and 

 Lassell), six to Uranus (of which the second and fourth are 

 most positively determined), and two to Neptune. 



The satellites revolving round the principal planets con- 

 stitute subordinate systems, in which the principal planets 

 take the place of central bodies, forming individual regions 

 of very different dimensions, in which the great solar region 

 is, as it were, repeated in miniature. According to our pres- 

 ent knowledge, the region of Jupiter is 208,000 geographical 

 miles in diameter, and that of Saturn 4,200,000. In Galileo's 

 * Outlines, 432. 



