6& 1'beory of [Book I. 



each tackle, the upper and the lower, parallel to each 

 other, to place them in a common neck, and to make 

 them move upon an axis common to all, as in fig. 6. 

 all the pullies are then of equal diameters. This kind 

 of tackle is in common ufe, efpecially on board mips. 

 The ropes, however, are not exactly parallels but this 

 defect is inconfiderable. 



In the preceding calculations the refiftance pro- 

 duced by friction, and that which arifes from the (tiff- 

 nefs and weight of the ropes are not regarded, on ac- 

 count of which it is neceflary to augment the power, 

 and to make it greater than I have fuppofed. It may 

 alfo happen, that in augmenting the number of pullies 

 thefe refiftances will be augmented fo much, that they 

 do more than compenfate for the augmentation of the 

 force which refults from the increafe of the number of 

 pullies. 



Wheels, like pullies 3 may be confidered as an afiem- 

 blage of levers. Of wheels there are two kinds : the 

 firft always turn in the fame fpace upon an axis fixed 

 to the center of the wheel, the .pivots of which turn 

 in holes or cavities which ferve as a prop: fuch are 

 the wheels of clocks, of mills, &c. Thefe kind of 

 wheels receive or tranfmit the movement by teeth, or 

 cogs placed round the circumference-. 



Wheels of the other kind, that is, which turn upon 

 their circumference, have their center or axle-tree in a 

 direction parallel to the plane on which they move ; 

 fuch are the wheels of waggons, coaches, &c. They 

 have therefore two movements j the one, of their 

 center which advances in a right line, and the other, 

 of all their parts which perform a circular motion 

 round the center. 



When wheels of the firft kind are put into action, 

 it is common to place upon the fame axle a great 



wheel 



