104 



WELLS S NATURAL PHILOSOPHY. 



Bi\v is poirer 

 gjiined at the 

 expense of 

 time in a sys- 

 tem of pul- 

 leys? 



In this we have four cords, one employed in sustaining the FiG. 83. 



power, P, and the other sustaining the weight; conse- 

 quently the power will be to the weiglit as 1 to 3. In 

 Fig. 84, we have two blocks, each containing two single 

 pulleys. The rope is thus divided into five portions, each 

 equally stretched; one is employed in supporting the 

 power P, and four sustam the weight. With this system 

 a power of 1 will balance a weight of 4. 



226. In all these arrangements of pul- 

 leys, the increase of power has been gained 

 at the expense of time, and the space 

 passed over by the power must be double 

 the space passed over by the weight, mul- 

 tiplied by the number of pulleys. That is, in the case of 

 the single pulley, the power must pass over two feet to 

 raise the weight one foot; and with two movable pulleys, 

 as in Fig. 84, the power must fall four feet to raise the 

 weight one foot. 



Instead of folding the string on the pulleys entire, it ia 

 sometimes doubled into separate portions, each pulley 

 hanging by a separate cord, one end of which is attached 

 to a fixed support. Here a very great mechanical advan- 

 tage is gained, attended, however, with a corresponding 

 loss of time. In an arrangement of such a character, re- 

 presented in Fig. 85, the weight W, is supported by the 

 two parts of the cord passing round the movable pulley, 

 C ; and as each of these parts is equally stretched, the 

 fixed support will sustain one half the weight, and the 

 next pulley in order above C, namely B, may be consid- 

 ered as sustaining the other half. But the two parts of 

 the string which support the pulley B, again divide the 

 weight, so that th -> pulley A, which is attached to one of 

 them, only sustains one quarter of the first weight, W. 



The string which passes around A again divides this 

 weight, so that each part of it sustains only one eighth 

 of W. The fixed pulley serves merely to change the 

 direction of the motion. In this system, therefore, a 

 power of 1 will balance a weight of 8. 



227. In general, the advan- 

 tage gained by pulleys is found 

 by multiplying the number of 

 movable pulleys by two, or by 

 multiplying the power by the number of 

 folds in the rope which sustains the weight, 

 where one rope runs through the whole. 



Fig. 85. 



JTow may the 

 .Tflvantige 

 enined by pul- 

 leys be ascer- 

 tained ? 



