56 MOVING POWERS IN MACHINERY. 



If two men work at the end of a roller, or wind- 

 lass, as in drawing up coals or ore from a mine, or 

 water from a well, they may more easily draw up 

 70 lbs. (still supposing the weight and power to 

 have equal velocities) than one man can 30 lbs., 

 provided the elbow of one of the handles be at 

 right angles to the other ; for then one man will 

 act at the strongest point when the other acts at 

 the weakest point of the revolution ; by which 

 means the two men will mutually and successively 

 help one another. A very common way is to put 

 on the handles opposite to one another, which 

 cannot give the advantage above-mentioned, though 

 there is some little force gained even in that posi- 

 tion, because one man pulling while the other 

 thrusts, works at the strongest of the two weak 

 points, whilst the other works at the weakest, and 

 so helps him a little. 



When a man carries a burden upon his back, he 

 exerts a great force very effectually, many muscles 

 being at once employed in that operation ; the 

 muscles of his neck, back, and loins, keep his body 

 and head in the proper position to sustain the 

 weight ; those of his shoulders and arms help to 

 keep it in its place ; and the muscles of his legs 

 and thighs raise the weight of all the body and 

 burden as the man walks along. In this way of 

 working, three men do much more than a horse, 

 and two often do as much, as may be observed in 

 the daily labour of the London porters. A porter 

 will carry 200 lbs. and walk at the rate of three 

 miles an hour ; a coal-heaver will carry 250 lbs. ; 

 but then he does not go far with his load. Chair- 

 men do not act with the same muscles as porters ; 

 but as they have straps brought down from their 

 shoulders to the poles of the chair, the muscles of 



