ASTRONOMY. 429 



the bodies which are included in our system ; that 

 is, the sun, with the primary and secondary planets. 

 But because the bulk of the sun greatly exceeds 

 that of all the planets put together, this point is in 

 the body of the sun. The attraction of the planets 

 on each other also somewhat disturbs their mo- 

 tions, and causes some irregularities. 



It is this mutual attraction between them and 

 the sun that prevents them from flying off from 

 their orbits by the centrifugal force which is gene- 

 rated by their revolving in a curve ; while the 

 centrifugal force keeps them from falling into the 

 sun by the force of gravity, as they would do if it 

 were not for this motion impressed upon them. 

 Thus these two powers balance each other, and 

 preserve order and regularity in the system. 



It has been already mentioned, (page 18) that 

 if, when a body is projected in a straight line, it 

 be acted upon by another force drawing it towards 

 a centre, it will be made to describe a curve, 

 which will be either a circle or an ellipsis, accord- 

 ing to the proportion between the projectile and 

 centripetal force. If a planet at B (Plate 23. 

 fig. 3.,) gravitates or is attracted towards the sun 

 S, so as to fall from B to g, in the time that the 

 projectile force would have carried it from B to X, 

 it will describe the curve B Y, by the combined 

 action of these two forces, in the same time that 

 the projectile force singly would have carried it 

 from B to X, or the gravitating power singly have 

 caused it to descend from B to y ; and these two 

 forces being duly proportioned, the planet obeying 

 them both, will move in the circle B Y T V. 



But if, whilst the projectile force would carry 

 the planet from B to b, the sun's attraction should 

 bring it down from B to U the gravitating power 



