AND MODERN PHYSICS. 171 



wheels' placed closo together, which we wish to turn 

 in the same direction. If the cogs can interlock, as in 

 Fig. 2, this is impossible : consecutive wheels in the 

 train must move in opposite directions. 



But in many machines the desired end is attained 

 by inserting between the two wheels A and B a third 

 idle wheel C, as shewn in Fig. 3. This may be very 



small, its only function is to transmit the motion of A 

 to B in such a way that A and B may both turn in the 

 sumo direction. It is not necessary that there should 

 bo cogs on the wheels; if the surfaces be perfectly 

 rough, so that no slipping can take place, the same 

 result follows without the cogs. 



Guided by this analogy Maxwell extended his 

 model by supposing each cell coated with a number 

 of small particles which roll on its surface. These 

 particles play the part of the idle wheels in the 

 machine, and by their rolling merely enable the 

 adjacent parts of two cells to move in opposite 

 directions. 



Consider now a number of such cells and their idle 

 wheels lying in a plane, that of the paper, and suppose 

 each cell is rotating with the same uniform angular 

 velocity about an axis at right angles to that plane, 

 each idle wheel will bo acted on by two equal and 

 opposite forces at the ends of the diameter in which 



