[495 3 
is made (or, as it is commonly called, the fall of 
the tackle) muft neceflarily be upon the outfide 
pulley or fhieve ; the difference of their fri&ion will 
give it fo great a tendency to pull the block awry, 
that as much will be loft by the rubbing of the 
fhieves againft the block, on account of its obliquity, 
as will be got by increafing the number of lines. 
The fecond method is free from this objection 5, 
but, as the length of the two blocks, taken together, 
muft be equal to the fum of the diameters of the fix 
pullies, befides the fpaces between for the ropes, and 
the neceftary appendages of the framing, were there 
more than three pullies in each block, they would 
run out into fuch an inconvenient length, as to de- 
duct very confiderably from the height, to which 
the weight might otherwife have been raifed : fo that, 
upon thofe accounts, no very great purchafe can be 
made by the common tackles of pullies alone. 
In order therefore to increafe its power, fometimes 
a fecond tackle is fixed upon the fall of the firft ; 
but here it is obvious, that whatever be the power 
of the fecond tackle, the height to which the weight 
might otherwife have been raifed by the firft, will be 
lefs in the fame proportion as the purchafe is in- 
creafed by the fecond. 
Again, very frequently the fall of the firft tackle 
is applied to an axis in peritrochio , which increafes 
the purchafe very commodioufly without the incon- 
veniencies laft-mention’d j but then the machine is 
render’d cumberfome, and, confequently, lefs fit for 
a moveable apparatus. 
All thofe impediments I have avoided, by com- 
bining the two methods, above defcribed, in one- 
The- 
