430 COMPAEATIVE LIGHT OF THE STARS. [SKCT. xxxvn. 



shortest period of any revolving star that we know of is 35 

 years. Still the distances of a vast number of stars may 

 ultimately be made out in this way ; and, as one important 

 discovery almost always leads to another, their masses may 

 thus be weighed against that of the earth or sun. 



The only data employed for finding the mass of the earth, 

 as compared with that of the sun, are, the angular motion 

 of our globe round the sun in a second of time, and the dis- 

 tance of the earth from the sun in miles (N. 224). Now, by 

 the observations of the binary systems, we know the angular 

 velocity of the small star round the great one ; and, when 

 we know the distance between the two stars on miles, it 

 will be easy to compute how many miles the small star 

 would fall through by the attraction of the great one in a 

 second of time. A comparison of this space with the space 

 which the earth would descend through towards the sun in 

 a second will give the ratio of the mass of the great star to 

 that of the sun or earth. According to M. Bessel, the weight 

 of the two stars of 61 Cygni is equal to half the weight of 

 the sun. Little as we know of the absolute magnitude of 

 the fixed stars, the quantity of light emitted by many of 

 them shows that they must be much larger than the sun. 

 Dr. Wollaston determined the approximate ratio which the 

 light of a wax candle bears to that of the sun, moon, and 

 stars, by comparing their respective images reflected from 

 small glass globes filled with mercury, whence a comparison 

 was established between the quantities of light emitted by 

 the celestial bodies themselves. By this method he found 

 that the light of a Lyrse is five and a half times greater than 

 that of the sun, that Sirius is nine times as bright as a Lyrse, 

 consequently, it gives forty-five times more light than the 

 sun, it is therefore estimated to be a hundred times as large ; 

 so that, were Sirius in the earth's place, its surface would ex- 

 tend 150 times as far as the orbit of the moon. The light 

 of Sirius, according to the observations of Sir John Herschel, 

 is 324 times greater than that of a star of the sixth mag- 



