esses it in the direction in which the steam flows. This end thrust is
counterbalanced by means of the "dummy" pistons, P^1, P^2, P^3.
Each dummy consists of a number of discs revolving between rings
projecting from the casing, the distance between discs and rings being
so small that but little steam can pass. In the high-pressure
compartment the steam pushes P^1 to the left with the same pressure as
it pushes the blades of D^1 to the right. After completing the first
stage it fills the passage C, which communicates with the second piston,
P^2, and the pressure on that piston negatives the thrust on D^2.
Similarly, the passage E causes the steam to press equally on P^3 and
the vanes of D^3. So that the bearings in which the shaft revolves
have but little thrust to take. This form of compensation is necessary
in marine as well as in stationary turbines. In the former the dummy
pistons are so proportioned that the forward thrust given by them and
the screw combined is almost equal to the thrust aft of the moving
vanes.

[Illustration: One of the turbine drums of the _Carmania_. Note the
rows of vanes. The drum is here being tested for perfect balance on two
absolutely level supports.]

ADVANTAGES OF THE MARINE TURBINE.

(1.) Absence of vibration. Reciprocating engines, however well balanced,
cause a shaking of the whole ship which is very unpleasant to
passengers. The turbine, on the other hand, being almost perfectly
balanced, runs so smoothly at the highest speeds that, if the hand be
laid on the covering, it is sometimes almost impossible to tell whether
the machinery is in motion. As a consequence of this smooth running
there is little noise in the engine-room--a pleasant contrast to the
deafening roar of reciprocating engines. (2.) Turbines occupy less room.
(3.) They are more easily tended. (4.) They require fewer repairs, since
the rubbing surfaces are very small as compared to those of
reciprocating engines. (5.) They are more economical at high speeds. It
must be remembered that a turbine is essentially meant for high speeds.
If run slowly, the steam will escape through the many passages without
doing much work.

Owing to its construction, a turbine cannot be reversed like a cylinder
engine. It therefore becomes necessary to fit special astern turbines to
one or more of the screw shafts, for use when the ship has to be stopped
or moved astern. Under ordinary conditions these turbines revolve idly
in their