J4 MOVING POWERS IN' .MACHINERY. 



in a straight line, a man exerts the least strength 

 that way; as, for example, if a man weighing 

 140 lbs., walking by a river or canal side, draws 

 along a boat, or barge, by means of a rope coming 

 over his shoulders, or otherwise fastened to his 

 body, he cannot draw above 9T( lbs., or about l-27th 

 of what a horse can draw in that case. Five men 

 are about equal in strength to one horse, and can 

 with the same ease push round the horizontal 

 beam in a 40 foot walk; but three of the same 

 men will push round a beam in a 19 foot walk, 

 which a horse, (otherwise equal to five men) can 

 but draw round. 



A man turning a horizontal windlass by a 

 handle, or winch, should not have above 30 lbs. 

 weight acting against him, if he is to work ten 

 hours a day, and raise the weight at the rate of 

 three feet and a half in a second. This supposes, 

 however, that the semi-diameter of the windlass is 

 equal to the distance from the centre to the elbow r 

 of the handle ; for if there be a mechanical advan- 

 tage, as there usually is, by having the diameter of 

 the axle on which the rope winds four or five times 

 less than the diameter of the circle described by 

 the hand, then may the weight (taking in also the 

 resistance, on account of the friction and stiffness 

 of the rope) be four or five times greater than 

 30 lbs. ; that is, so much as it rises slower than the 

 hand moves. 



In this operation, the effect of a man's force 

 varies in every part of the circle described by the 

 handle. The greatest force is, when a man pulls 

 the handle upwards from about the height of his 

 knees ; and the least force when (the handle being 

 at top) he thrusts from him horizontally ; then 

 again the effect becomes greater, as a man lays on 

 his weight to push down the handle ; but that 



