Chap. 7.] rhe Pulley., Ci 



ley, that is the hollow part of the circumference which 

 receives the cord is generally hollowed out angularly 

 and not round, fo that the cord being in fome meafure 

 pinched or compreffed in this angle, ir will not be liable 

 to glide or flip in its motion. 



Pullies are commonly made of wood or metal, and 

 always turn upon an axis. When they are made of 

 wood, it is better to fix the axis to the pulley, and to 

 let all turn together in the fpace which fu Mains the 

 pulley. The movement then being performed upon 

 a lefs furface will be lefs impeded by friction, and if 

 the fpace which contains the pulley becomes larger, as 

 it is only the lower part which can be affected, the 

 aperture will be lengthened, the pulley will defcend a 

 little, but its circular motion will not be diminiihed; 

 it is not fo when the pulley turns upon its center, for 

 then if the aperture which receives the axis enlarges, 

 the enlargement is frequently not equal in all its 

 parts. 



By means of pullies burdens are elevated with 

 greater eafe, and in a more commodious manner than 

 they otherwife could be ; more commodious becaufe 

 the motion is continued, 'and its direction may be 

 changed fo as to bring the whole force which is ap- 

 plied to it into immediate action, for by this means a 

 horfe which can only exert his force in an horizontal 

 direction, is able to overcome a vertical refiftance. 

 Burdens are moved with more eafe by pullies, becaufe 

 a great weight may be elevated by a fmall force pro- 

 perly applied. The power applied to a pulley draws 

 in all directions without impediment, becaufe the cord 

 by which it acts is always a tangent * to the circum- 



* A tangent is a right line drawn perpendicular from the ex- 

 tremity of the radius, which touches the circumference of a circle 

 without cutting it. 



ference 



