MOVING POWERS IN MACHINERY. 55 



action cannot be so great as when he pulls up, 

 because he lays on no more than the whole weight, 

 of his body ; whereas, in pulling, he can exert his 

 whole strength. Lastly, he has but small force to 

 pull the handle towards him horizontally, when at 

 its lowest. 



Let us suppose a man of moderate strength to 

 weigh 140 lbs. ; he may in the four principal parts 

 of pushing and pulling, in the whole circumference 

 of motion, exert the following forces ; viz. in the 

 strongest point, a force equal to 160 lbs. ; in the 

 weakest, a force equal to '27 lbs. ; in the next strong 

 point, ISO lbs. ; and in the last, or second weak 

 point, 30 lbs. Let us add all these forces together, 

 which will make 347 ; which divide by 4, and we 

 shall have 84| lbs. for the weight that a man might 

 lift by a winch, if he could exert his whole force 

 continually, without stopping to take breath ; but 

 as that cannot be, the weight must return, and 

 overpower at the first weak point, especially when 

 the handle moves slowly, as it must, if a man would 

 exert his whole strength all round. Besides, for 

 raising such a weight, we must suppose the man 

 acting always along the tangent of the circle of 

 motion, which does not happen in the operation. 

 Then there must be a sufficient velocity given, 

 that the force applied at the strong points may not 

 be spent before the hand comes to the weak ones, 

 so that it is difficult for a man to continue that 

 irregular motion ; and, therefore, when there are 

 no other advantages, the resistance ought to be but 

 30 lbs. If a fly be added to the windlass when the 

 motion is pretty quick, as about four or five feet 

 in a second, a man may for a little while act with 

 a force of 80 lbs. and work a whole day with a re- 

 sistance of i'O lbs. 



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