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6In these machines the blocks are constructed differently from those already described. Having, at top and bottom, two ranks of pulleys, the rope passes through a hole in the lower block, so that each end of the rope is equal in length when extended. It is there bound and made fast to the lower block, and both parts of the ropes so retained, that neither of them may swerve either to the right or the left. The ends of the rope are then returned to the outside of the upper block, and carried over its lower pulleys; whence they descend to the lower block, and passing round its pulleys on the inner side, are carried up right and left over the tops of the higher pulleys of the upper block;
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