140 



THEORY OF NEWTONIAN FORCES. [PT. I. CH. III. 



Suppose that the coordinate y and the velocity a? a ' are constant. 

 If now #/ is increased, say by raj starting from rest and moving 

 to the right by the application of a positive force P Xi , then P^ is 

 positive if y\< d/Z and m 3 is within the rails, in other words, 

 unless a force to the right is impressed on m z also, # 2 ' will diminish, 

 and if a?/ was also zero, ra 2 will move to the left. 



The force P Xz must be greater the smaller \y\. This is the 

 analogue of the induction of currents. Similar effects may be 

 produced by moving m 3 along the rod, instead of applying a 

 force to m l or ra 2 . 



FIG. 30. 



Maxwell's model, which undoubtedly suggested Thomson's, 

 differs from it only in having motion of rotation instead of *eve- 

 -, so that there is no limit to the possible difference in the 

 coordinates sc lt a? 2 . The independent masses are represented by 

 the moments of inertia of masses m 1? m 2 carried by two shafts 

 S lt $2, Fig. 30, each of which carries a bevel-gear wheel A, B 

 at one end. Engaging these is a pair of bevel-gears C running 

 loosely upon a third perpendicular shaft, carrying the inter- 

 mediate mass, ra 3 . 



If all the bevel-gears are of the same diameter, and fa, </> 2 , <f> 3 

 are the angles made by the three horizontal rods with a fixed 

 horizontal line, then it is evident, since the velocity of the centre 

 of the wheel C is a mean between the velocities of its highest 

 and lowest points, which have respectively the velocities of the 

 rims of the wheels A and B, that 



+ ' 



