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.