THE PULLEY. 



275 



all systems of pulleys of this class, the weight of the lower block is to be con- 

 sidered as a part of the weight to be raised, and, in estimating the power of 

 the machine, this should always be attended to. 



When the power of the machine, and therefore the number of wheels, is 

 considerable, some difficulty arises in the arrangement of the wheels and 

 cords. The celebrated Smeaton contrived a tackle, which takes its name 

 from him, in which there are ten wheels in each block : five large wheels 

 placed side by side, and five smaller ones similarly placed above them in the 

 lower block, and below them in the upper. Fig. 9 represents Smeaton'e blocks 



without the rope. The wheels are marked with the numbers 1,2, 3, &c., in 

 the order in which the rope is to be passed over them. As in this pulley, 

 twenty distinct parts of the rope support the lower block, the weight, including 

 the lower block, will be twenty times the equilibrating power. 



In all these systems of pulleys, every wheel has a separate axle, and there 

 is a distinct wheel for every turn of the rope at each block. Each wheel is 

 attended with friction on its axle, and also with friction between the sheave 

 and block. The machine is by this means robbed of a great part of its efficacy, 

 since, to overcome the friction alone, a considerable power is in most cases 

 necessary. 



An ingenious contrivance has been suggested, by which all the advantage 

 of a large number of wheels may be obtained without the multiplied friction of 

 distinct sheaves and axles. To comprehend the excellence of this contrivance, 

 it will be necessary to consider the rate at which the rope passes over the sev- 

 eral wheels of such a system, as fig. 7. If one foot of the rope G F pass over 

 the pulley F, two feet must pass over the pulley E, because the distance be- 

 tween F and E being shortened one foot, the total length of the rope G F E 

 must be shortened two feet. These two feet of rope must pas 3 in the direc- 

 tion E D ; arid the wheel D, rising one foot, three fee': of rcpe musi conse- 

 quently pass over it. These three feet of rope passing in the direction D C, 

 and the rope D C being also shortened one foot by the ascent of the lower 

 block, four feet of rope must pass over the wheel C. In the same way it may 

 be shown that five feet must pass over B, and six feet over A. Thus, what- 

 ever be the number of wheels in the upper and lower blocks, the parts of the 

 rope which pass in the same time over the wheels in the lower block are in 

 the proportion of the odd numbers 1, 3, 5, &c. ; and those which pass over the 



