122 .IAMKS rl.KHK MAXWKLL 



more accurate, <lo not differ among themselves by 

 luoro than some small quantity. The number of 

 people at any moment in each of these groups will 

 be very different. The number in any group, which 

 has a velocity not differing greatly from the mean 

 velocity of the whole, will be large ; comparatively few 

 will have either a very large or a very small velocity. 



Again, at any moment, individuals are changing 

 from one group to another; a man is brought to 

 a stop by some obstruction, and his velocity is con- 

 siderably altered he passes from one group to a 

 different one; but while this is so, it' the mean velocity 

 remains constant, and the sixe of the crowd be very 

 great, the number of people at any moment in a 

 given group remains unchanged. People pass from 

 that group into others, but during any interval tin- 

 same number pass back again into that group. 



It is clear that if this condition is satisfied the 

 distribution is a steady one, aiul the crowd will continue 

 to move on with the same uniform mean velocity. 



Xow, Maxwell applies these considerations to a 

 crowd of perfectly elastic spheres, moving any how in 

 a closed space, acting upon each other only when 

 in contact. He shows that they may be divided into 

 groups according to their velocities, and that, when 

 the steady state is reached, the number in each group 

 will remain the same, although the individuals change. 

 Moreover, it is shown that, if A and B represent any 

 two groups, the state will only be steady when the 

 numbers which pass from the group A to the group 

 B are equal to the numbers which pass back from the 

 group B to the group A. This condition, combined 



