Solar System* 45 



regular indeed, that their phenomena 

 may be pretty exactly calculated for ages. 

 There may be other planets, which hu- 

 man observation has not yet discovered, 

 and which, perhaps, it never may. 



All the planets move nearly in the di- 

 rection of the ecliptic ; and that space 

 on each side of it, which bounds their ut- 

 most deviations, is called the Zodiac. It 

 is a broad circle or belt in the starry 

 heavens ; and is about 16 degrees in 

 breadth, and the ecliptic equally divides 

 it in two all round, so that the planets are 

 always found amongst some of the con- 

 tellations, which form the twelve signs of 

 the zodiac. 



The reason of their motions in curve 

 lines, is the attraction of the sun, or their 

 gravitations towards the sun (call it 

 which you please ;) and an oblique or 

 side-long impulse or motion ; the former 

 is commonly called the centripetal force , 

 and the latter the centrifugal force. 



These two motions or tendencies, the 



one always endeavouring to carry them 



ma straight line from the circle they move 



in, and the other endeavouring to draw 



D2 



