146 ON THE ORIGIN OF THE PLANETARY SYSTEM. 



its end a small heavy body for instance, a lead ball. 

 If you allow this to hang at rest, it stretches the 

 thread. This is the position of equilibrium of the 

 ball. To indicate this, and keep it visible, put in 

 the place of the ball any other solid body for in- 

 stance, a large terrestrial globe on a stand. For this 

 purpose the ball must be pushed aside, but it presses 

 against the globe, and, if taken away, it still tends to 

 come back to it, because gravity impels it towards its 

 position of equilibrium, which is in the centre of the 

 sphere. And upon whatever side it is drawn, the same 

 thing always happens. This force, which drives the 

 ball towards the globe, represents in our model the 

 attraction which the earth exerts on the moon, or the 

 sun on the planets. After you have convinced your- 

 selves of the accuracy of these facts, try to give the 

 ball, when it is a little away from the globe, a slight 

 throw in a lateral direction. If you have accurately 

 hit the strength of the throw, the small ball will 

 move round the large one in a circular path, and may 

 retain this motion for some time ; just as the moon 

 persists in its course round the earth, or the planets 

 about the sun. Now, in our model, the circles 

 described by the lead ball will be continually narrower, 

 because the opposing forces, the resistance of the air, 

 the rigidity of the thread, friction, cannot be elimi- 

 nated, in this case, as they are excluded in the plane- 

 tary system. 



