Water,
from its excessive mobility, is incapable of giving any resistance to
the screw or paddle save that due to its inertia. If, for example, we
conceive of the existence of a sea without any inertia, then we can
readily understand that the water composing such a sea would offer no
resistance to being pushed astern by paddle or screw. When a gun is
fired, the weapon moves in one direction--this is called its
recoil--while the shot moves in another direction. The same
principal--_pace_ Professor Greenhill--operates to cause the movement
of a ship. The water is driven in one direction, the ship in another.
Now, Professor Rankine has laid down the proposition that, other
things being equal, that propeller must be most efficient which sends
the largest quantity of water astern at the slowest speed. This is a
very important proposition, and it should be fully grasped and
understood in all its bearings. The reason why of it is very simple.
Returning for a moment to our gun, we see that a certain amount of
work is done on it in causing it to recoil; but the whole of the work
done by the powder is, other things being equal, a constant quantity.
The sum of the work done on the shot and on the gun in causing their
motions is equal to the energy expended by the powder, consequently
the more work we do on the gun, the less is available for the shot.
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