VOL. XLVII.] PHILOSOPHICAL TRANSACTIONS. 279 



bhx^ks, taken together, must be equal to the sum of the diameters of the 6 

 pulleys, besides the spaces between for the ropes, and the necessary appendages 

 of the framing, when more than 3 pulleys are in each block, they run out into 

 ^ich an inconvenient length, as to deduct very considerably from the height to 

 which the weight might otherwise have been raised : so that, on those accounts, 

 no very great purchase can be made by the common tackles of pulleys alone. 



In order therefore to increase its power, sometimes a 2d tackle is fixed on 

 the fall of the first ; but here it is obvious, that whatever be the power of the 

 2d tackle, the height to which the weight might otherwise have been raised by 

 the first, will be less, in the same proportion, as the purchase is increased by 

 the 2d. 



Again, very frequently the fall of the first tackle is applied to an axis in peri- 

 trochio, which increases the purchase very commodiously without the inconve- 

 niences last-mentioned ; but then the machine is rendered cumbersome, and con- 

 sequently less fit for a moveable apparatus. 



All those impediments Mr. S. has avoided, by combining the two methods, 

 above described, in one. The pulleys are here placed in each block in 2 tiers ; 

 several being on the same pin as in the first method, and every one having 

 another under it, as in the 2d; as also that, when the tackle is in use, the 2 

 tiers that are the most remote from each other, are so much larger in diameter 

 than those that are nearest, as to allow the lines of the former to go over the 

 lines of the latter without rubbing. 



From this construction arises a new method of reeving the line on the shieves : 

 for here, whatever be the number of shieves, the fall of the tackle will always be 

 on the middle shieve, or on that next the middle, according as the number of 

 pulleys on each pin is odd or even. 



To do this, the line is fixed to some convenient part of the upper block, and 

 brought round the middle shieve of the larger tier of the under block, from 

 thence round one of the same sort next to the centre one of the upper block ; 

 and so on till the line comes to the outside shieve, where the last line of the 

 larger tier falls on the first shieve of the smaller, and being reeved round those, 

 till it comes at the opposite side, the line from the last shieve of the smaller tier 

 again rises to the first of the larger, whence it is conducted round till it ends on 

 the middle shieve of the upper block on the larger tier ; as will appear more 

 plain, by inspection of figure 5, pi. 7- 



In this method all the lines are clear of one another, and the blocks are kept 

 parallel. The model which he showed the Society, and from which he made the 

 draught, is a composition of 20 shieves, 5 on each pin. With this model, 

 which may easily be carried in the pocket, he had raised 600 weight. But with 

 a tackle of this sort, properly executed in large, one man will easily raise a ton. 



