AND THEORY OF THE HEAVENS. 8 1 



The planets are formed out of particles which at the 

 distance at which they move, have exact movements in 

 circular orbits ; and therefore the masses composed out of 

 them will continue the same movements at the same rate 

 and in the same direction. This suffices to show why the 

 motion of the planets is almost circular, and why 

 their orbits are in one plane. They would -be exact 

 circles 1 if the range out of which they collected the 

 elements for their formation was very small, and then 

 the difference of their movements would be very slight. 

 But as a wide range of space is required in order that 

 the dense mass of a planet may be formed out of the 

 fine stuff which is so widely diffused in the celestial 

 space, the difference of the distances which these 

 elements have from the sun, and consequently also 

 the difference of their velocities, is no longer in- 

 significant. In consequence, it would be necessary, in 

 order to maintain in the planet the equilibrium of the 

 centripetal forces and the circular velocity with this 

 difference of movements, that the particles which come 

 together upon it from different distances with different 

 movements should exactly compensate for the defects 

 of each other. And although this really takes place 

 with tolerable exactness, 2 nevertheless, as it falls some- 



1 This exact circular movement properly applies only to the planets 

 near the sun ; for at the great distances where the most remote 

 planets, or at least the comets, have been formed, it is easy to 

 suppose that, as the falling movement of the elementary stuff was 

 much feebler there, the extension of the space in which they were 

 diffused was also greater, and that the elements there already 

 diverge of themselves from the circular movement and must thereby 

 be the cause of the bodies formed from them. 



2 For the particles in the region nearer the sun, which have a 

 greater velocity of revolution than is required for the circular 



