MOTION,, MECHANICS, ETC. 153 



you will feel only half the weight, the cord attached to the 

 hook supporting the other half ; if you put the cord over 

 the fixed pulley, this only changes the direction, and, there- 

 fore, in pulling the cord downwards, you only feel half the 

 weight ; one ounce suspended from the cord passing over 

 the fixed pulley, will balance two ounces from the movable 

 pulley, and you will be enabled to lift twice as much weight 

 by the assistance of a single movable pulley, as you could 

 raise by your own actual strength. The power moves twice 

 as fast as the pulley with the weight ; therefore, the space 

 described by the power will be equal to twice the space 

 described by the weight, or the velocity of the weight is to 

 that of the power as one to two ; and it will be observed 

 that when the weight has been raised two inches, the power 

 has descended four inches. 



c. When the upper and fixed block contains two pulleys, 

 which only turn on their axis, and the lower movable block 

 contains also two, which not only turn on their axis, but 

 rise with the block and weight, the advantage gained is as 

 four to one. 



D. When there are three pulleys in the fixed block, and 

 three in the movable one, D, and the number of cords six, 

 the power is as six to one ; the advantage is the same whe- 

 ther the pulleys are placed parallel to or under each other. 



E. In this arrangement the pulleys do not, as in the pre- 

 ceding systems, rise together in one block with the weight, 

 but act upon one another, so that each pulley doubles the 

 power. A power of one ounce will be in equilibrio with 

 two ounces at the first movable pulley, with four at the 

 second, with eight at the third ; and the velocity of the 

 power will be eight times that of the weight. 



White's pulley described in the first set of mechanic pow- 

 ers is sometimes added to this set. 



The Wheel and Axis (Fig. 159, page 151) is a machine 

 much used, and which is applied in a variety of forms. The 

 power acts on the circumference of the wheel ; the weight 

 is fastened to one end of a cord or rope, whose other end 

 winds round an axis that turns with the wheel. The axis, 

 F, and wheel, G, in Fig. 159, are fastened together so that 

 one cannot move without the other ; when a weight, w, is 

 to be raised by this engine, it is fixed to the end that goes 

 round the axis, but the power, p, is applied to one of the 



