;8 IN STARRY REALMS. 



making this computation, so that the result thus obtained 

 is rather in excess of what would have been the result if 

 we had retained several small elements in the calculation 

 which we have thought it convenient to discard in such an 

 elementary sketch. The final result, as obtained by the 

 most trustworthy methods, is that it would take 326,800 

 earths, all rolled into one, to form a mass as ponderous 

 as the sun. 



It will thus be seen from this illustration, that the 

 observation of satellites in attendance upon a planet 

 enables us to weigh that planet. Hence we see that the 

 discovery of the satellites of Mars and the observation of 

 their periods, conducted us to an accurate knowledge of 

 the mass of the ruddy planet himself. I say an accurate 

 knowledge, for it must not be presumed that we did not 

 know something of the weight of this planet before. The 

 different departments of astronomy are so closely inter- 

 woven, that the influence of each planet is often felt under 

 circumstances that would hardly be credited by one who 

 had not the means of following out the complete details of 

 the subject. Our earth, in pursuing its annual progress 

 round the sun, is, of course, mainly guided by the all- 

 compelling and powerful attraction of the great central 

 luminary. But every planet also draws the earth in some 

 degree. The capacity of the planet to make the earth 

 swerve to the side of its path is dependent on the mass of 

 the planet. If the planet be great, its influence will be 

 correspondingly large. By incessant observations of the 

 sun, astronomers have learned, with the utmost minute- 

 ness of detail, the nature of the path which the earth 

 follows. They are actually able to disentangle the mar- 



