422 COSMOS. 



distinguished from the secondary planets, or satellites, by 

 larger type. Some bodies are included in the class of prin- 

 cipal planets, which form a peculiar and very extended group, 

 forming, as it were, a ring of 132 millions of geographical 

 miles, situated between Mars and Jupiter, and are generally 

 called small planets, as well as telescopic planets, co-planets, 

 asteroids, or planetoids. Of these, 4 were discovered in 

 the first seven years of this century, and 10 during the last 

 six years ; which latter circumstance is to be attributed less 

 to the perfection of the telescopes, than the industry and 

 dexterity of the investigators, and especially the improved 

 charts, enlarged by additions of fixed stars of the ninth and 

 tenth magnitudes. It is now more easy to distinguish 

 between moving cosmical bodies and fixed. (See Cosmos, vol. 

 iii. p. 155.) 



The number of the principal planets has been exactly 

 doubled since the first volume of Cosmos appeared, 17 so 

 excessively rapid is the succession of discoveries, the exten- 

 sion and perfection of the topography of the planetary system. 



2. Classification of the planets in two groups. If the region 



and the time of its first announcement. In consequence of 

 a neglect of this distinction, dissimilar and erroneous dates 

 have been introduced into astronomical manuals. So, for 

 example, Huygens discovered the sixth satellite of Saturn 

 (Titan) on March 25, 1655 (Huy genii Opera varia. 1724, 

 p. 523), and did not announce it until March 5, 1656 (Sys- 

 tema Saturnium, 1659. p. 2). Huygens, who devoted himself 

 uninterruptedly from March, 1655, to the study of Saturn, 

 had already obtained the full and indubitable view of the 

 open ring on December 17, 1657. (Systema Safurnium, p. 

 21), but did not publish his scientific explanation of all 

 the phenomena until the year 1659. (Galileo had thought 

 that he saw, on each side of the planet, only two projecting 

 circular discs.) 



17 Cosmos, vol. i. p. 76. Compare also Encke in Schu- 

 macher's Astron. Nachr., vol. xxvi. 1848, No. 622. p. o ? 47. 



