OJf PRESSURE AND EQUILIBRIUM. GQ 



to support the moveable block, each of them bearing a weight equivalent to 

 the force \yhich is applied at the end of the thread. In the common ship's 

 blocks, the pullies or shieves are equal in magnitude, and placed side by side ; 

 here their number cannot conveniently exceed two or three, without causing 

 an obliquity in the block, when the force is applied to the rope. Mr. Smeaton, 

 for this reason, imeuted a system of pullies, arranged in two rows in each 

 block, one larger, and the other smaller : the force being applied in the mid- 

 dle, the rope passes on the larger pullies, till it arrives at the last, then re- 

 turns through the whole of the smaller series, to the opposite side, and comes 

 back again on the larger, to be finally attached in the middle. (Plate IV. 

 Fig. 54: .. 56.) 



If the diameters of all the pullies, in both blocks, be taken in the ratio of 

 the number of portions of the thread intervening between them and the fixed 

 extremity, their angular velocity will be equal, each of them turning on its 

 axis in the same time. They may therefore be fixed to a single axis in each 

 block ; and in this case the axis being longer, there will be less accidental 

 friction from its want of steadiness, and even the necessary friction may, per- 

 haps, be somewhat diminished. (Plate IV. Fig. 57. ) 



If one end of a thread} supporting a moveable pulley, be fixed, and the other 

 attached to another moveable pulley, and tlic threads of this pulley be similarly 

 arranged, the weight will be counterpoised by a power, which is found by 

 halving it as many times as there are moveable pullies; for it is obvious that 

 each of the&e pullies doubles the efilxt of the power. (Plate IV. Fig. 58.) 



There are also other arrangements, by which the eflfect of pullies may be in- 

 creased or diversified : for instance, where one end of each rope is attached 

 to the weight to be moved; or where two of the pullies are connected by a 

 rope passing over a third; but these methods are of little practical utility. 

 (Plate IV. Fig. 59, 60.) 



We have hitherto supposed the ropes, passing over the pullies, to be either 

 perfectly or very nearly parallel to each other ; but when their directions are 

 oblique, the forces applied to them require to be modified accordingly. Thus, 

 if two threads be attached to a weight, and passed over two pullies, fixed at a 



