Chap. XT.

MECHANICS.

93S

Fig. 12G4.

Suppose the pulleys in eachblock to be n, then while theweight ascends an inch, thepower descends 2 n inches; andconsequently, when there isequilibrium, the power is tothe weight as 1 to 2 n.

By mariners the case whichcontains the wheel or sheeve ofthe pulley is called the block ;two or more blocks with therope constitute a tackle.

When each block containssix sheeves, the ropes are madeto pass from one to the other,as expressed in the section, andthe power is to be calculatedin the ordinary way.

There are many systems ofpulleys which are worked by asingle rope ; in all of this kindthere is one movable block inwhich wheels or sheeves arefixed over which the rope runs,and to which the weight isattached: the weight here israised by the Iow'er block, andthe rope, instead of being finallyattached to the fixed block attop, passes over a second wheelin that block, and then under asecond in the lower. Theweight here is equally distri-buted among four ropes, eachof which is stretched by the force of the power; hence in this case the weight is four timesthe power.

When passed over the third wheel itsincrease of power may be estimated in asimilar manner. M is the fixed point;the weight W is suspended on the sheevewhich contains the three lower wheelscomprised within R S, as the three upper .

are between M and N: the rope after f\passing over A B and under V T, rises toC, and then descends and follows its course,as represented in the former example.

In Whites pulley, all the wheels in thesame block are reduced to one; or rather,instead of using separate wheels, severalgrooves are cut round circles of differentdiameters upon the same wheel: as allcircles are proportional to their diameters,it follows that such wheels come under thesame law ; in the lower block of a pulleyso contrived the diameters of the severalcylinders are as the odd numbers, 1, 3, 5, . v

7, 9, &c. ; whilst those on the upper block, 1 V

which is fixed, are as the even ones, 2, 4, 6, ^ VJ

8, &c. : in such an arrangement the uni-

versal law of mechanics is followed, viz.that as force is diminished, velocity in-creases ; or, the power must pass over aspace greater than that gone over by theweight, by as many times as the weight Fig. 1266. Fig. 1267. Fig . 12G8.

exceeds the power.

Whites pulley is so arranged, that the successive wheels revolve in the same timealthough they be of different diameters, and are so constructed that their several circum

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