efore it has imparted some of its _velocity_ to the
water, which thus gains sufficient momentum to force down the valve and
find its way to the boiler. The overflow space O O between E and C
allows steam and water to escape until the water has gathered the
requisite momentum.

[Illustration: FIG. 15.--Diagram illustrating the principle of a
steam-injector.]

[Illustration: FIG. 16.--The Giffard injector.]

A form of injector very commonly used is Giffard's (Fig. 16). Steam is
allowed to enter by screwing up the valve V. As it rushes through the
nozzle of the cone A it takes up water and projects it into the "mixing
cone" B, which can be raised or lowered by the pinion D (worked by the
hand-wheel wheel shown) so as to regulate the amount of water admitted
to B. At the centre of B is an aperture, O, communicating with the
overflow. The water passes to the boiler through the valve on the left.
It will be noticed that the cone A and the part of B above the orifice O
contract downward. This is to convert the _pressure_ of the steam into
_velocity_. Below O is a cone, the diameter of which increases
downwards. Here the _velocity_ of the water is converted back into
_pressure_ in obedience to a well-known hydromechanic law.

An injector does not work well if the feed-water be too hot to condense
the steam quickly; and it may be taken as a rule that the warmer the
water, the smaller is the amount of it injected by a given weight of
steam.[2] Some injectors have flap-valves covering the overflow orifice,
to prevent air being sucked in and carried to the boiler.

When an injector receives a sudden shock, such as that produced by the
passing of a locomotive over points, it is liable to "fly off"--that is,
stop momentarily--and then send the steam and water through the
overflow. If this happens, both steam and water must be turned off, and
the injector be restarted; unless it be of the _self-starting_ variety,
which automatically controls the admission of water to the
"mixing-cone," and allows the injector to "pick up" of itself.

For economy's sake part of the steam expelled from the cylinders of a
locomotive is sometimes used to work an injector, which passes the water
on, at a pressure of 70 lbs. to the square inch, to a second injector
operated by high-pressure steam coming direct from the boiler, which
increases its velocity sufficiently to overcome the boiler pressure. In
this case only a fraction of the weight of high-