FRICTTON IN MECHANICS. 383 



PROPOSITION' 5. THEOREM. 



In the application of forces to overcome friction, the 

 fame allowances muft be made for the forces acting to 

 advantage or di (advantage, by means of levers or other 

 mechanical powers, as are made in the common doc- 

 trine; forinftance, if a weight of two pounds, by acting 

 at the diftance of one foot from the fulcrum of a lever, 

 be fufficient to overcome the friction, then one pound 

 at two feet diftance will have the fame effect, Sec. 



This is too evident to need a dcmonftration. 



OF FRICTION' IN THE SCREW. 



Asanv force acting perpendicular to the direction of 

 a moving body does not affect the motion of the body 

 in that direction, fo the force acting perpendicular to the 

 axis of the ferew, has no effect on the motion of a body 

 raifed thereby, exclu five of friction; it therefore requires 

 the fame force to raife a body by means of a fcrew, as to 

 raife the fame body in equal time along an inclined plane 

 of the fame elevation, as the threads of the fcrew by 

 means of a force acting parallel to the bafe of the in- 

 clined plane : now. if we fuppofe the weight fo con- 

 tracted or condenfed as to be capable of being placed 

 on one of the threads of the fcrew, and fattened to an 

 imaginary lever always perpendicular to its axis, then it 

 is evident this lever will have no effect but to change 

 the direction of the weight, and keep it in the midft of 

 the thread of the fcrew ; and if a force be applied at 

 the weight alwavs perpendicular to this lever, fo as to 

 fuftain or draw it along, this force will be determined 

 exactly the fame as was done before in the inclined 

 plaae : but the rigidity of the parts of the " female 

 § fcrew" 



