A TURBINE WORKS.

In reciprocating--that is, cylinder--engines steam is admitted into a
chamber and the door shut behind it, as it were. As it struggles to
expand, it forces out one of the confining walls--that is, the
piston--and presently the door opens again, and allows it to escape when
it has done its work. In Hero's toy the impact of the issuing molecules
against other molecules that have already emerged from the pipes was
used. One may compare the reaction to that exerted by a thrown stone on
the thrower. If the thrower is standing on skates, the reaction of the
stone will cause him to glide backwards, just as if he had pushed off
from some fixed object. In the case of the _reaction_--namely, the
Hero-type--turbine the nozzle from which the steam or water issues
moves, along with bodies to which it may be attached. In _action_
turbines steam is led through fixed nozzles or steam-ways, and the
momentum of the steam is brought to bear on the surfaces of movable
bodies connected with the shaft.

THE DE LAVAL TURBINE.

In its earliest form this turbine was a modification of Hero's. The
wheel was merely a pipe bent in S form, attached at its centre to a
hollow vertical shaft supplied with steam through a stuffing-box at one
extremity. The steam blew out tangentially from the ends of the S,
causing the shaft to revolve rapidly and work the machinery (usually a
cream separator) mounted on it. This motor proved very suitable for
dairy work, but was too wasteful of steam to be useful where high power
was needed.

[Illustration: FIG. 36.--The wheel and nozzles of a De Laval turbine.]

In the De Laval turbine as now constructed the steam is blown from
stationary nozzles against vanes mounted on a revolving wheel. Fig. 36
shows the nozzles and a turbine wheel. The wheel is made as a solid
disc, to the circumference of which the vanes are dovetailed separately
in a single row. Each vane is of curved section, the concave side
directed towards the nozzles, which, as will be gathered from the
"transparent" specimen on the right of our illustration, gradually
expand towards the mouth. This is to allow the expansion of the steam,
and a consequent gain of velocity. As it issues, each molecule strikes
against the concave face of a vane, and, while changing its direction,
is robbed of its kinetic energy, which passes to the wheel. To turn
once more to a stone-throwing comparison, it is as if a boy were pelting
the wheel with