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. 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.