MOTION, MECHANICS, ETC. 147 



In the best instruments there are seven systems of pulleys, 

 viz. 1. a fixed pulley with one wheel or sheave, turning on 

 pivots by a cord ; 2. one fixed and one movable pulley con- 

 nected ; 3. two fixed pulleys in one block, and two movable 

 pulleys in one block ; 4. two fixed pulleys in one block, the 

 one above the other, and two movable pulleys one above 

 the other ; 5. three pulleys arranged as the last in both the 

 moving and fixed pulley ; 6. a fixed pulley and four single 

 pulleys, having the cord supported from a fixed point ; 7. 

 White's pulley, which consists of a fixed pulley, having five 

 or more concentric grooves on the face of the wheel, with 

 diameters, as 1, 3, 5, and one movable pulley with another 

 such wheel, having corresponding grooves with diameters, 

 as 2, 4, 6, on the lower block; the cord being passed in suc- 

 cession over the grooves, as represented in the cut, it will be 

 thrown off by the action of the power in the same manner 

 as if every groove formed a separate and independently re- 

 volving wheel. 



It is to be observed in using the pulleys, that the pulley 

 is to be balanced before the weights, representing the 

 power, are applied ; for this purpose a cup with hooks is 

 supplied, into which shot is placed till the pulley is accu- 

 rately balanced. 



Price, with levers, screw, inclined plane, wheel 



and axis, and set of brass weights, - $35.00. 



The Levers. The principal lever (Fig. 153, opposite page) 

 is a bar of brass about twelve inches long, supported on a 

 steel pivot, called the fulcrum, at about four inches from 

 one end, which end is made thicker than the longer one, 

 that they may equally balance. On the under side of the 

 lever is a succession of hooks, at one inch from the fulcrum, 

 and from each other, for the whole length ; and on the up- 

 per side of the lever a succession of hooks, on the longer 

 side only. The lever is attached by its fulcrum to the upper 

 part of the frame supporting the pulleys. Over the lever 

 is a frame supported by two brass pillars, having a sliding 

 pulley, which may be placed in* such a manner as to carry 

 the cord from any of the hooks on the upper side of the 

 lever, as weuld be required in using the lever in the second 

 and third order. On the top of the frame there is a series 

 of three levers, the shorter arm of the one acting on the 

 longer arm of the next, forming a compound lever. Also, 



