SOLAR SYSTEM. 153 



as the motion of a body revolving about a larger 

 one by means of a central force. Perhaps the 

 reader may understand what kind of adjustments 

 these are, by supposing such a bowl and ball to 

 be used for a game of skill. If the object of the 

 players be to throw the pellet along the surface 

 of the basin, so that after describing its curved 

 path it shall pass through a small hole in a 

 barrier at some distance from the starting point, 

 it will easily be understood that some nicety* in 

 the regulation of the force and direction with 

 which the ball is thrown will be necessary for 

 success. In order to obtain a better image of the 

 solar system, we must suppose the basin to be 

 very large and the pellet very small. And it 

 will easily be understood that as many pellets as 

 there are planets might run round the bowl at 

 the same time with different velocities. Such a 

 contrivance might form a planetarium in which 

 the mimic planets would be regulated by the 

 laws of motion as the real planets are ; instead of 

 being carried by wires and wheels, as is done in 

 such machines of the common construction : and 

 in this planetarium the tendency of the planets 

 to the sun is replaced by the tendency of the re- 

 presentative pellets to run down the slope of the 

 bowl. We shall refer again to this basin, thus 

 representing the solar system with its loose 

 planetary balls. 



