The circular system is developed continuously over the entire subumbral surface, and the velum represents a special local development of this system, at a region where it is able to act at the greatest mechanical advantage in producing the contractions of the umbrella by which the animal progresses.
The value d/D usually lies between the limits io/i i and 15/16, and, if a greater difference of E from R is required, a further mechanical advantage can be obtained by employing a separate hand-wheel and chain, or by forming the upper sheave with an annular spurwheel gearing with a pinion driven by a hand-wheel and chain, as in the Tangye form of Weston pulley-block.
The worm-wheel shaft then sometimes carries a spur-pinion gear ing with a spur-wheel on the lifting shaft, whereby a much greater mechanical advantage is obtained with a small loss by friction of the spur gearing.
