stance show that one propeller must be more powerful than
the other?

_A._--That does not follow necessarily, nor is it the fact in this
particular case. All steam vessels when set into motion, will force
themselves forward with an amount of thrust which, setting aside the loss
from friction and from other causes, will just balance the pressure on the
pistons. In a paddle vessel, as has already been explained, it is easy to
tell the tractive force exerted at the centre of pressure of the paddle
wheels, when the pressure urging the pistons, the dimensions of the wheels
and the speed of the vessel are known; and that force, whatever be its
amount, must always continue the same with any constant pressure on the
pistons. In a screw vessel the same law applies, so that with any given
pressure on the pistons and discarding the consideration of friction, it
will follow that whatever be the thrust exerted by a paddle or a screw
vessel, it must remain uniform whether the vessel is in motion or at rest,
and whether moving at a high or a low velocity through the water. Now to
achieve an equal speed during calms in two vessels of the same model, there
must be the same amount of propelling thrust in each; and this thrust,
whatever be its amount, cannot afterward vary if a uniform pressure of
steam be maintained. The thrusts, therefore, caused by their respective
propelling instruments, when a screw and paddle vessel are tied stern to
stern, must be the same as at other times; and as at other times those
thrusts are equal, so must they be when the vessels are set in the
antagonism supposed.

578. _Q._--How comes it then that the screw vessel preponderates?

_A._--Not by virtue of a larger thrust exerted by the screw in pressing
forward the shaft and with it the vessel, but by the gravitation against
the stern of the wave of water which the screw raises by its rapid
rotation. This wave will only be raised very high when the progress of the
vessel through the water is nearly arrested, at which time the centrifugal
action of the screw is very great; and the vessel under such circumstances
is forced forward partly by the thrust of the screw, and partly by the
hydrostatic pressure of the protuberance of water which the centrifugal
action of the screw raises up at the stern.

579. _Q._--Can you state any facts in corroboration of this view?

_A._--The screw vessel will not preponderate if a screw and paddle vessel
be tied bow to bow an