MECHANICS. 



93. T. How is this accomplished? 



P. The weight (W) hangs in the cord, 

 and drags down each side of it equally 

 . so that it has the same strain 

 upon it at C as at P, consequently the 

 weight (W) is divided between the two 

 ends of the cords ; which is the same as 

 saying, that if the weight (W) is forty 

 pounds, the end C of the cord sustains 

 twenty pounds, and the end P the other 

 twenty pounds. 



94. T. What is the principle upon 

 which pulleys act? 



ing of three fixed, and three movable pul- 

 leys, the weiht of which, attached to 

 the common block of the three movable 

 pulleys, is supported by the six lines which 



[Lesson VII. 



connect the upper and lower pulleys ; and, 

 therefore, as the weight is equally divided 

 between the lines, each is drawn by one- 

 sixth of the weight (</). Consequently, if 

 sixty pounds weight is suspended to the 

 bottom, each line would be drawn upon by 

 a force of ten pounds. If we wish to keep 

 this machine in a state of equilibrium, we 

 must attach a weight (p) of ten pounds to 

 the end of the line. Suppose that a per- 

 son had to raise sixty pounds weight with 

 this machine, it is evident that, by pulling 

 at p with a force equal to ten pounds, this 

 would be accomplished ; therefore, we see 

 the great advantage gained by employing 

 this machine. 



97. T. It appears that the diminution 

 of weight is in proportion to the number 

 of pulleys used. Is this the case? 



P. Yes ; and if we wish to calculate 

 the expenditure of power or decreast of 

 weight, we must multiply the number of 

 movable pulleys by 2, and the product is 

 the power required to be exerted. Thus, 

 3 movable pulleys multiplied by 2, gives 

 6 ; from which we know that a sixth of 

 the weight is the power we require. 



98. T. What other varieties are there 

 of a combination of pulleys? 



P. We may have a 

 system of pulleys with 

 different cords so ar- 

 ranged, as to act succes- 

 sively on one another; 

 indeed, the variety 01 

 tackles of pulleys is al- 

 most endless. 



99. T. Can you give 

 me an example of a 

 tackle which does away 

 with much of the irre- 

 gularity of action and 

 friction, so common to 

 pulleys generally? 



P. Yes ; a machine 

 called White's pulley 

 obviates much of these 

 defects. It is composed 

 of a proper number of 

 concentric grooves cut 

 on the face of a solid 

 wheel, with diameters, 

 Fi ' 28- as the odd numbers 1, 



3, 5, 7, &c., for the lower block, and cor- 

 responding grooves on another such wheel, 



