166 ON THE OEiaiN OF THE PLANETARY SYSTEM. 



in the motion of the planets since the time in which 

 we possess astronomical observations of their path. It 

 is different with the smaller bodies of our system. 

 Encke in particular has shown, with reference to the 

 well-known small comet which bears his name, that it 

 circulates round the sun in ever-diminishing orbits and 

 in ever shorter periods of revolution. Its motion is 

 similar to that of the circular pendulum which we have 

 mentioned, and which, having its velocity gradually 

 delayed by the resistance of the air, describes circles 

 about its centre of attraction, which continually become 

 smaller and smaller. The reason for this phenomenon 

 is the following: The force which offers a resistance 

 to the attraction of the sun on all comets and planets, 

 and which prevents them from getting continually 

 nearer to the sun, is what is called the centrifugal 

 force that is, the tendency to continue their motion 

 in a straight line in the direction of their path. As 

 the force of their motion diminishes, they yield by a 

 corresponding amount to the attraction of the sun, and 

 get nearer to it. If the resistance continues, they will 

 continue to get nearer the sun until they fall into it. 

 Encke's comet is no doubt in this condition. But the 

 resistance whose presence in space is hereby indicated, 

 must act, and has long continued to act, in the same 

 manner on the far larger masses of the planets. 



The presence of partly fine and partly coarse 



