de which the
drum revolves; (4) the vanes on the drum and casing; (5) the balance
pistons. Fig. 37 shows a diagrammatic turbine in section. The drum, it
will be noticed, increases its diameter in three stages, D^1, D^2,
D^3, towards the right. From end to end it is studded with little
vanes, M M, set in parallel rings small distances apart. Each vane has a
curved section (see Fig. 38), the hollow side facing towards the left.
The vanes stick out from the drum like short spokes, and their outer
ends almost touch the casing. To the latter are attached equally-spaced
rings of fixed vanes, F F, pointing inwards towards the drum, and
occupying the intervals between the rings of moving vanes. Their concave
sides also face towards the left, but, as seen in Fig. 38, their line of
curve lies the reverse way to that of M M. Steam enters the casing at A,
and at once rushes through the vanes towards the outlet at B. It meets
the first row of fixed vanes, and has its path so deflected that it
strikes the ring of moving (or drum) vanes at the most effective angle,
and pushes them round. It then has its direction changed by the ring of
F F, so that it may treat the next row of M M in a similar fashion.

[Illustration: FIG. 38.--Blades or vanes of a Parsons turbine.]

[Illustration: One of the low-pressure turbines of the _Carmania_, in
casing. Its size will be inferred from comparison with the man standing
near the end of the casing.]

THE EXPANSIVE ACTION OF STEAM IN A TURBINE.

On reaching the end of D^1 it enters the second, or intermediate, set
of vanes. The drum here is of a greater diameter, and the blades are
longer and set somewhat farther apart, to give a freer passage to the
now partly expanded steam, which has lost pressure but gained velocity.
The process of movement is repeated through this stage; and again in
D^3, the low-pressure drum. The steam then escapes to the condenser
through B, having by this time expanded very many times; and it is found
advisable, for reasons explained in connection with compound
steam-engines, to have a separate turbine in an independent casing for
the extreme stages of expansion.

The vanes are made of brass. In the turbines of the _Carmania_, the huge
Cunard liner, 1,115,000 vanes are used. The largest diameter of the
drums is 11 feet, and each low-pressure turbine weighs 350 tons.

BALANCING OF THRUST.

The push exerted by the steam on the blades not only turns the drum, but
pr