DOUBLE AND MULTIPLE STARS. 137 



to many thousands of years in 66 Ceti, 38 Geminorum, and 

 100 Piscium. In the triple system of Cancri, the nearest 

 companion of the principal star has already more than accom- 

 plished one entire revolution since HerscheFs measurement in 

 1782. By means of skilful combinations of the changes of 

 distance and of angles of position ( 104 ), the elements of the 

 orbits have been assigned, and conclusions have even been 

 drawn respecting the absolute distance of the double stars 

 from the earth, and their mass as compared with the mass 

 of the Sun. But whether the attracting forces depend solely 

 on the quantity of matter in these systems as in ours, or 

 whether there may not coexist with gravitation other specific 

 forces which do not act according to mass, is, as Bessel has 

 been the first to shew, a question of which the solution is 

 reserved for later ages( 105 ). 



If we desire to compare our Sun with others of the 

 fixed stars or self-luminous suns, within the lenticular side- 

 real stratum to which we belong, we find that in the case of 

 some of them at least, there are methods by which we may 

 arrive approximately, and within certain limits, at a know- 

 ledge of their distance, their volume, their mass, and the 

 velocity of their motions in space. If we take the 

 distance of Uranus from the Sun at 19 times the solar dis- 

 tance of the Earth, then the central body of our system is 

 11900 such spaces, or solar distances of Uranus, from 

 a Centauri, 31300 from 61 Cygni, and 41600 from a Lyra. 

 The comparison of the volume of the Sun with the volume 

 of stars of the first magnitude is dependent on an optical 

 element which is subject to extreme uncertainty, viz. the 

 apparent diameter of the fixed stars. If, with Herschel, we 

 assume the apparent diameter of Arcturus even at only the 



