NEWTON S PRINCIPIA. 157 



even in his day there prevailed considerable uncertainty 

 respecting this planet s mass, while that of Jupiter, being 

 well ascertained, agrees perfectly with Sir Isaac Newton s 

 deduction. Laplace gives the masses of the four great 

 planets thus, that of the Sun being unity : Venus SJ ^Q^ ; 

 Mars 25^20; Jupiter lQ ^ g (differing by T V only 

 from Newton s, who indeed did 4 not insert decimals at all) ; 

 and Saturn _ A_* The Moon s mass he makes l -, 



o 5 3 4 U a o a o 7 



that of the Earth being unity, while the greatest of 

 Jupiter s satellites is only 0,0000884972, Jupiter being 

 unity. This great geometrician s observations upon Sa 

 turn s ring are peculiarly worthy of attention. The ex 

 treme lightness of the matter of which the planet consists, 

 has already been shown ; it is six times lighter than the 

 mean density of the Earth; or, if the mean specific 

 gravity of the latter be taken as 5 f, that of water being 

 as 1, the matter of which Saturn is composed must be 

 only 3J times heavier than cork, and lighter than India 

 rubber. But Laplace has satisfactorily shown that his 

 rings must be composed of a fluid, and that no other con 

 struction can account for their permanence.^ 



3. Sir Isaac Newton, lastly, by the principles which we 

 have been explaining in the latter part of our Analysis, 

 investigated the figures of the heavenly bodies. Thus he 

 especially examined that &quot;of the Earth. This planet, in 

 revolving round its axis, gives those particles the greatest 

 tendency to fly off which move with the greatest velocity, 

 that is, those which are furthest from their centres of ro 

 tation ; in other words, those which are nearest the equa 

 tor ; while those near the poles, describing much smaller 

 circles, move much slower and have far less tendency to 



* Mec. CeL liv. x. ch. 8, 9 ; correcting liv. vi. ch. 6. 

 f The mean of Maskelyue aud Cavendish s experiments. 

 Mec. CeL liv. iii. ch. 6. 



