PHYSICAL ASTRONOMY, 



SECT. III. 



DISTURBANCES IN THE MOTIONS OF THE PRIMARY 

 PLANETS PRODUCED BY THEIR ACTION' ON ONE 

 ANOTHER, 



IT is necessary, in this inquiry, to know the quanti- 

 ties of matter in the different planets ; and these 

 have been already calculated for the planets which 

 have satellites. The masses of Venus and Mars 

 have been computed by M. LA PLACE from some 

 disturbances which they appear to produce on the 

 Earth's motion. The mass of Mercury has been 

 estimated, from supposing his density, and that of 

 the Earth, to be inversely as their mean distances 

 from the Sun. This law holds with respect to 

 the Earth, Jupiter and Saturn, and analogy au- 

 thorises the extension of it to Mercury. From 

 knowing the density and the bulk, the quantity of 

 matter is inferred. The mass of the Sun being 1, 



that of Mercury is , of Venus 



* 2025810 38313?' 



of Mars ] 84?60 gg those of the other being as 

 already stated, 24-5* 



The 



