5 S The Lfirr. [Book I. 



lias been prtfoppoTcd that powers acting upon it have 

 cither been in perpendicular directions, or in thole of 

 equal obliquity to the arms of the lever. 



The moil advantageous pofition for a power act- 

 ing by means of a lever is that which is perpendicu- 

 lar to the arm of the lever. As in the inftance of the 

 lever (fig. 7.) if the power B acts in the direction b B, 

 it will exert the greateft polTible force ; but it will 

 have lefs force if it acts in the directions b D or b E. 

 When, however, one power is oblique to the arm of 

 the lever, and another power becomes equally oblique 

 by being in parallel directions, as the lines a p and b r 

 (fig. 8) then they have the fame relation as before 

 to each other. But if the directions are in different 

 degrees of obliquity, then that which departs moft 

 from a right angle will have the lead force ; for exam- 

 ple, if the power Q^(lig. 9.) prefer ves a perpendicu- 

 lar direction, and the or,her power an oblique one, act* 

 ing upon the line p c, pd> p e> orp/, the latter power 

 then becomes weaker in proportion as it departs from 

 the perpendicular direction p P. 



From what has been already dated, it follows that 

 the power is greater, or lefs, or equal to the refiftance, 

 according as the diftance of the refiftance from the 

 prop is greater, or lefs, or equal to that of the power. 

 Hence in a lever of the firft order, the power may 

 be either greater, or lefs, or equal to the refiftance ; 

 in a lever of the fe'cond order, the power is always 

 lefs than the refiftance j and laftly, it follows that it 

 muft be greater in a lever of the third order : fo that 

 this order of lever, fo far from aiding the power as to 

 its abfolute force, muft, on the contrary, impede it. 

 Yet it is the lever of the third order which nature moft 

 frequently employs in the human body *. Thus when 



* See Borelli de motu aninialium. 



we 



