It then shuts
up like an umbrella, and assumes a volume so small that its resistance on
the return journey is insignificant. After passing round the drum at the
upper end, it at once opens afresh of its own accord, and once more
becomes part of the moving power of the whole system. The parachutes are
formed by first cutting out a complete circle of cloth, and then taking
from this a sector equal to one-fifth or one-sixth of the total area. Such
parachutes are found to keep their form when stretched by the water better
than a surface originally spherical, although the latter would be
theoretically more correct. The motion of the drum is transmitted by spur,
gear, or otherwise as may be required, to give the requisite speed.
It will be seen that the advantages of the system are as follows: First,
the facility it offers for obtaining a large working area, which may be
increased or diminished at will, according to the requirements of the
moment, by lengthening or shortening the rope. Secondly, the ease with
which it is erected and set to work. Thirdly, the small part of the river
section which it occupies, so as to present no obstacle to navigation.
Fourthly, the ease with which it can be mounted on a barge of any kind,
and carried wherever it may be needed.
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