2O8 CELESTIAL MEASURINGS AND WEIGHINGS. 



if all the ponderable matter but a single pound were col- 

 lected in one of them, and that pound circulated about 

 it as the earth does about the sun. Here, then, we have 

 the case stated over again, with only the difference of 

 times and distances, which, in our Lecture on '* The 

 Sun," already referred to, 16, 17, served us to show how 

 we might arrive at a knowledge of the sun's mass, and to 

 calculate that mass. Substitute in the reasoning there 

 explained for one year 78 or 514 years, and for the sun's 

 distance respectively fifteen and thirty times that distance ; 

 and the result, in place of the mass of the sun, will fur- 

 nish us with the total or joint masses of the two stars in 

 the one or the other of these two sidereal combinations or 

 "binary systems" respectively. We shall not trouble 

 our readers with the calculation : suffice it to state the 

 result, viz., that the joint mass in question in the former 

 pair (that in the Centaur), is about &, a little more 

 than half that of the sun, or equal to 198,000 earths; 

 and in the latter (in the Swan), about ^ of the sun, 

 equivalent to 36,000 earths. 



(34.) Beyond the distances of these two remarkable 

 sidereal combinations, our grasp becomes less and less 

 assured as we push forward into space. Remarkably 

 enough, Sirius and Arcturus, the two brightest stars 

 visible in our hemisphere, stand barely within the limits 

 of any estimation approaching to certainty, the former 

 being between six and seven, the latter about eight, 

 times the distance of our nearest neighbour in the Cen- 

 taur. At the distance thus assigned to Sirius, our sun (if 

 any faith can be placed in photometry) would appear as 



