138 COSMOS. 



Sun with the other fixed stars that is, with other self-lumi- 

 nous Suns in the lenticular starry stratum of which our sys- 

 tem forms a part, we find, at least in the case of some, that 

 channels are opened to us, which may lead, at all events, to 

 an approximate and limited knowledge of their relative dis- 

 tances, volumes, and masses, and of the velocities of their 

 translatory motion. If we assume the distance of Uranus 

 from the Sun to be 19 times that of the Earth, that is to say, 

 19 times as great as that of the Sun from the Earth, the cen- 

 tral body of our planetary system will be 11,900 times the 

 distance of Uranus from the star a in the constellation Cen- 

 taur, almost 31,300 from 61 Cygni, and 41,600 from Vega in 

 the constellation Lyra. The comparison of the volume of 

 the Sun with that of the fixed stars of the first magnitude is 

 dependent upon the apparent diameter of the latter bodies, 

 an extremely uncertain optical element. If even we assume, 

 with Herschel, that the apparent diameter of Arcturus is only 

 a tenth part of a second, it still follows that the true diameter 

 of this star is 1 1 times greater than that of the Sun.* The 

 distance of the star 61 Cygni, made known by Bessel, has 

 led approximately to a knowledge of the quantity of mat- 

 ter contained in this body as a double star. Notwithstanding 

 that since Bradley's observations, the portion of the apparent 

 orbit traversed by this star, is not sufficiently great to admit 

 of our arriving with perfect exactness at the true orbit and the 

 major axis of this star, it has been conjectured with much pro- 

 bability by the great Konigsberg astronomer,! " that the m ^ss 

 of this double star cannot be very considerably larger or 

 smaller than half of the mass of the Sun." This result is from 

 actual measurement. The analogies deduced from the rela- 

 tively larger mass of those planets in our solar system that 

 are attended by satellites, and from the fact that Struve has 

 discovered six times more double stars amongst the brighter 



S. 2-6. The question has been raised by John Tobias Mayer, in Com- 

 ment. Soc. Reg. Gotting., 1804-1808, vol. xvi. pp. 31-68. 



* Philos. Trans, for 1803, p. 225. Arago, in the Annuaire, 1842, p. 

 375. In order to obtain a clearer idea of the distances ascribed in a 

 rather earlier part of the text to the fixed stars, let us assume that the 

 Earth is a distance of one foot from the Sun ; Uranus is then 19 feet, and 

 Vega Lyree is 158 geographical miles from it. 



f Bessel, in Schum. Jakrb., 1839, s. 53. 



