It
can be shown that, if the gun weighed no more than the shot, when the
charge was ignited the gun and the shot would proceed in opposite
directions at similar velocities--very much less than that which the
shot would have had had the gun been held fast, and very much greater
than the gun would have had if its weight were, as is usually the
case, much in excess of that of the shot. In like manner, part of the
work of a steam engine is done in driving the ship ahead, and part in
pushing the water astern. An increase in the weight of water is
equivalent to an augmentation in the weight of our gun and its
carriage--of all that, in short, takes part in the recoil.
But, it will be urged, it is just the same thing to drive a large body
of water astern at a slow speed as a small body at a high speed. This
is the favorite fallacy of the advocates of hydraulic propulsion. The
turbine or centrifugal pump put into the ship drives astern through
the nozzles at each side a comparatively small body of water at a very
high velocity. In some early experiments we believe that a velocity of
88 ft. per second, or 60 miles an hour, was maintained. A screw
propeller operating with an enormously larger blade area than any pump
can have, drives astern at very slow speed a vast weight of water at
every revolution; therefore, unless it can be shown that the result is
the same whether we use high speed and small quantities or low speed
and large quantities, the case of the hydraulic propeller is hopeless.
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