CIRCULAR ORBITS. 155 



pellet so that after running half round the surface 

 it should touch a point exactly at an equal dis- 

 tance from the centre, on the other side, passing 

 neither too high nor too low. And the pellet, it 

 may be observed, should be in size only one ten 

 thousandth part of the distance from the centre, 

 to make the dimensions correspond with the case 

 of the earth's orbit. If the mark were set up and 

 hit we should hardly attribute the result to chance. 



The earth's orbit, however, is not exactly a 

 circle. The mark is not precisely a single point, 

 but is a space of the breadth of one thirtieth of 

 the distance from the centre. Still this is much 

 too near an agreement with the circle to be con- 

 sidered as the work of chance. The chances 

 were great against the ball passing so nearly at 

 the same distance, for there were twenty-nine 

 equal spaces through which it might have gone, 

 between the mark and the centre, and an in- 

 definite number outside the mark. 



But it is not the earth's orbit alone which is 

 nearly a circle : the rest of the planets also 

 approach very nearly to that form : Venus more 

 nearly still than the earth : Jupiter, Saturn, and 

 Uranus have a difference of about one-tenth, 

 between their greatest and least distances from 

 the sun : Mars has his extreme distances in the 

 proportion of five to six nearly ; and Mercury in 

 the proportion of two to three. The last mentioned 

 case is a considerable deviation, and two of the 



