Complex sentence: Wire ropes attached to both sides of each caisson passed over the pulleys to 36 cast iron counterweights weighing 14 tons, 18 on each side to balance the 252 ton weight of a loaded caisson.
Split into simpler sentences: Wire ropes attached to each side of each caisson passed over the pulleys to 36 cast iron counterweights, 18 on each side. Each counterweight weighed 14 tons, so that 18 counterweights would exactly balance the 252 ton weight of a loaded caisson.