494 PHYSIOLOGICAL PHYSICS. [Chap. XLI. 



will = 12 x 4 48. If the force cc be applied in 

 the opposite direction, as indicated by the line cc', 

 then the forces Act, B&, and cc', will be in equilibrium. 

 The two forces tending downwards will be counter- 

 balanced by a single force in the opposite direction, 

 applied at the point c. This shows that cc is the 

 resultant of Act and B&. 



A couple. Two unequal parallel .forces in op- 

 posite directions can also be reduced to a single force 

 acting in the direction of the greater force and equal 

 in amount to the difference between the two forces. 

 When two equal and parallel forces are opposite they 

 have no resultant, and there is no single force which 

 can balance them. This is called a COUPLE, and it 

 tends to produce a movement of rotation. 



CHAPTER XLI. 



THE LEVER, PULLEY, AND BALANCE. 



THE principles that have been explained are exem- 

 plified in certain simple machines, the lever, etc. 



The lever is simply an application of the facts 

 of parallel forces. This is evident from Fig. 203, which 



represents a rigid bar 

 under the influence 

 of two forces, p and 

 w. F is the fixed 



Fig. 203.-Lever of the First Order. 



Now the tendency of the force w to pull the bar 

 down towards it is measured by its moment, i.e. its 

 amount multiplied by the distance between its point 

 of application and the point of application of F. 

 Suppose w to be the force of a 10-pound weight, and 



