328 MARKET DAIRYING 
tampered with. Both can be set to blow off at different 
pressures. 
Water Feed Apparatus. There are two ways ct 
feeding water into a boiler, namely, with injectors and 
with pumps. 
Injector. This important boiler accessory is attached 
to the side of the boiler. It utilizes the steam 
directly from the boiler for forcing water into it 
against a pressure as great as that which sends it forth. 
The principle which makes this possible may be stated 
as follows: Steam issuing from a boiler under 70 pounds 
pressure has a velocity of 1,700 feet per second. When 
steam with this high velocity strikes the combining tube 
it produces suction which in turn induces a flow of 
water. As soon as the water enters the combining tube 
it is given motion by the high velocity of the steam, 
which immediately condenses and moves with the water 
into the boiler at a comparatively low velocity. The 
energy, therefore, by which steam can force water into the 
boiler against its own pressure is the latent heat resulting 
from the condensation of the steam in the combining tube. 
From this it must be evident that the efficiency of the 
injector is dependent upon the completeness with which 
the steam condenses. This is clearly proven by every 
day practical experience. When, for instance, the feed 
water is too hot, the steam pressure too high, or the 
steam is wet, the injector fails to work properly because 
the steam does not sufficiently condense when it strikes 
the feed water. 
Starting the Injector: This is done by opening the 
supply water valve one or two turns, then the steam valve 
wide. If steam issues from the overflow admit a little 
more water ; if water overflows admit less, 
