The two return pulleys are, however,
connected by a line, E, which can be shortened or lengthened from the
pontoon, and in this way the angle of inclination between the two ropes
can be varied if required. A grooved pulley presses upon the trailing span
at the moment before it reaches the circumference of the drum. It is
mounted on a screwed spindle, which is depressed by a nut, and thus makes
the wet rope grip the outside of the drum in a thoroughly efficacious
manner.
The author has made a theoretical investigation of the power which may be
developed by the system, and has worked out tables by which, when the
velocity of the current and the other elements of the problem are known,
the power developed by any given number of parachutes can be at once
determined. We do not reproduce this investigation, which takes account of
the resistance of the returning parachutes and other circumstances, but
will content ourselves with quoting the final equation, which is as
follows: T = 0.328 S V cubed. Here T is the work done in H.P., S is the total
working area in sq. m., and V is the velocity of the current in m. per
sec. Taking V = 1, and S = 1 sq. m., which is by no means an impracticable
quantity, we have T = 0.328 H.P. per sq. m. We may check this result by
the equation given, in English measures, by Rankine--"Applied Mechanics,"
p.
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