It is found that when open and in motion the parachutes
never touch the bottom. This was the case with a rope containing 180
parachutes of 4 feet diameter, and working in a depth of only 6 feet. This
is easily explained by the fact that the velocity of a current always
diminishes as it approaches the bottom. Hence the pressure on the lower
part of the parachute will be less than that on the upper part; but the
former pressure tends to draw the parachute downward, while the latter
tends to raise it to the top of the water. Thus, the latter being the
larger, the parachute will always have a tendency to rise. In fact, it is
necessary to sink the return pulley sufficiently deep to make sure that
the parachutes will not emerge from the surface. For the same reason no
intermediate supports are needed over the driving span; if any are needed
it is for the return span, on which the parachutes are closed. Of course,
if metal were used instead of hemp, the case would be entirely different,
and intermediate supports would have to be used for anything but very
moderate lengths.
In practice, Mr. Jagn has employed two ropes wound upon the same pair of
drums, which are mounted upon a pontoon. The ropes are spread out from
each other, as in Fig.
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