[Fig. 105.] Three Fold Purchase using a six part line, gives a theoretical mechanical advantage of 6:1 and an actual advantage of 5·03:1, assuming a loss of 5% of the force on the rope passing over each sheave.
[Fig. 106.] Four Fold purchase using two four-sheave blocks, is commonly used in derricks and hoists. The illustration shows the possibility of using four two-sheave blocks, where the larger sizes are not available.
[Fig. 107.] The Double Burton (A), for one rope and two single blocks and one double block, gives a limited hoisting range which is desirable in shifting heavy weights when it is necessary to lift them but a small distance.
[Fig. 108.] The Double Burton (B), while using exactly the same equipment as is used in [Fig. 91], shows the large differences in mechanical advantage between different methods of threading up the set. The illustration also shows a box sling in use.
[Fig. 109.] Double Burton (C), is a further application of the principle of the Spanish Burton, using two ropes.