AND MOhKKN PHYSICS. 121 



pie ted in 1850, a\nd in which ho had shown that the 

 conditions of stability required the supposition that 

 the rings are composed of an indefinite number of free 

 particles revolving round the planet, with velocities 

 depending on their distances from the centre. These 

 particles may either be arranged in separate rings, or 

 their motion may be such that they are continually 

 coming into collision with each other. 



As an example of the statistical method, let us 

 consider a crowd of people moving along a street. 

 Taken as a whole the crowd moves steadily forwards. 

 Any individual in the crowd, however, is jostled back- 

 wards and forwards and from side to side ; if a line 

 were drawn across the street we should find people 

 crossing it in both directions. In a considerable in- 

 terval more people would cross it. going in the direc- 

 tion in which the crowd is moving, than in the other, 

 and the velocity of the crowd might bo estimated 

 by counting the number which crossed the line in 

 a given interval. This velocity so found would differ 

 greatly from the velocity of any individual, which 

 might have any value within limits, and which is 

 continually changing. If wo knew the velocity of 

 each individual and the number of individuals we 

 could calculate the average velocity, and this would 

 agree with the value found by counting the resultant 

 number of people who cross the line in a given in- 

 terval. 



Again, the people in the crowd will naturally full 

 into groups according to their velocities. At any 

 moment there will be a certain number of people 

 whose velocities are all practically equal, or, to be 



