43 THE PULLEY. 



is. The elbow being the centre round which the lower part 

 of the arm turns, the muscle, therefore, must exert a force 

 ten times as great as the weight to be raised. At first view 

 this may appear a disadvantage, but the loss of power is com- 

 pensated by the gain of velocity, and by the beauty and com- 

 pactness of the limb. 



Questions. — 1. What are raechauical powers? 2. What four 

 things are necessary to be considered in order to understand the pow- 

 er of a n>achine ? 3. When do the power and w^ght balance each 

 other ? 4. Wljat is a lever ? 3. Describe the lever oTthe first kind. 6. 

 What are some instances of it, and to what purposes are they applica- 

 ble ? 7. What is said of a bala|)ice ? 8. Describe the second kind of 

 4evcr. 9. What does it explainr*? JO. What is the third kind of lever .' 

 H. Show ho'^v^^e bones of a maaPs arm make a lever of this kind. 

 12. How is thJfcoss of power compensated ? ^3. Give an illustration 

 by fig. 7. of the first kind of lever. 14. Of the secondkind, bvfigure 

 9 and 5. 15. Of the third kind) hgr figures 10 and 2. .^ #^' - 



i 



The Pulley, Wheel cQm Axle, anarWnclined Plane 



The pulley is formed by a sniall wheel, t^%/^ of wood or 

 metal, with a groove in its circumference, which is placed in 

 a frame and turns on an axis. The wheel is usually called 

 a sheeve, and is so fixed in the frame, or block, as to move 

 round a pin passing through its centre. Pullies are of two 

 kinds ; Jixed, which do not move out of their places ; atid 

 moveable, which rise and fall with the weight. A single fixed 

 pulley gives no mechanical advantage, but it is of great im- 

 portance in changing the direction of power, and is much 

 used in buildings for drawing up small weights, for a man 

 may raise a weight to any height without the fatigue of as- 

 cending a ladder. In the single moveable pulley, the advan- 

 tage gained is as two to one ; that is, a power exerted by the 

 hand of ten pounds will balance a weight of twenty pounds. 

 In a system of pullies, the power gained nmst be estimated, 

 by doubling the number of pullies in the lower or moveable 

 block. So that when the fixed block contains two pullies 

 which only turn on their axes, and the lower block also con- 

 tains two, which not only turn on tlieir axes, but rise with 

 the weight, the advantage gained is as four to one. In an 



