PORTION OF THE COSMOS. THE PLACETS. 391 



actual discovery was favoured by the excellent star-maps of 

 the Berlin Academy, by Bremiker ( 637 ). If, among the 

 distances of the outer planets from the Sun, the distance of 

 Saturn (9 '53) is approximately twice as great as that of 

 Jupiter (5 -20),. and the distance of Uranus (19 -18) approxi- 

 mately twice as great as that of Saturn, the distance of 

 Neptune (30 '04) would require, in order to complete a 

 similar proportion, to have a third part more, or fully ten 

 Earth-distances, added to it. Our planetary boundary is at 

 the present time 621 millions of German, or 2484 millions 

 of English, geographical miles from the central body ; by 

 the discovery of Neptune, the terminal or border- stone, 

 marking the limit of our planetary knowledge, has been 

 made to recede more than 223 millions (892 English) such 

 miles, or upwards of 10 '8 distances of the Earth from the 

 Sun. Step by step as the perturbations suffered by each 

 last-known planet are recognised, fresh and fresh planets are 

 discovered, until, by reason of their remoteness, they cease 

 to be visible through our telescopes ( 638 ). 



According to the latest determinations the period of revo- 

 lution of Neptune is 60126-7 days, or 164 years and 226 

 dajs; and its semi-major axis 30'03628. The excentricity 

 of its orbit is 0-00871946, the least next to that of Venus; 

 its mass is l - f T6 : its apparent diameter is, according 

 to Encke and Galle, 2"' 70, and according to Challis even 

 3"-07 ; which would give its density as compared to that 

 of the Earth 0'230, greater therefore than that of Uranus 

 0-178 (639). 



Soon after the discovery of Neptune a ring was ascribed 

 to it by Galle, Lassell, and Challis. The first-named of 

 these astronomers had employed a magnifying power of 567, 



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