50 ELEMENTS OF SCIENCE 



lever, the inclined plane, and the pulley, referring him 

 for further explanation about them to professed treatises 

 on mechanics. 



To return, for a little to the consideration of the 

 action of two parallel and unequal forces : 



Let us suppose that a heavy rigid bar is balanced on a 

 point at its centre, which must, of course, be its centre 

 of gravity. 



Now if we suppose the bar to be made up of two 

 bars, a larger one, AD, and a shorter one, DB, then 

 they also can be supported at their respective centres 

 namely, at D' for AD, and at D" for DB. But the 

 two bars thus respectively supported at D' and D", act 

 as two parallel and unequal forces (namely, the weight 

 of each), and their resultant must pass through the point 



FIG. 7. 



A D 1 C D D" B 



A 



Y 



C, because it is at that point that their two pressures 

 are neutralised by the support which balances the 

 whole. Hence we see that the resultant of the two 

 unequal forces does not pass through a point midway 

 between them (i.e., mid-way between D' and D"), but 

 through the point C, which is much nearer to D' than 

 to D". It is just so much nearer as the weight of AD 

 is greater than the weight of BD. That is to say, as 

 D'C : D"C : : the weight of BD : the weight of AD. 

 In other words, the resultant of the two unequal 

 parallel forces is so situated that its distance from 

 either, shall be inversely as their intensities. To prove 

 the truth in the example chosen, we may suspend from 



