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476 Canadian Record of Science. 
cooling vessels where the fluid, by remaining for some time 
before suffering further compression and consequent heating 
in the next cylinder, can part with some of its heat. Thus, 
by the action of the inter-coolers, the pressures against 
which the piston in the second and third cylinders have to 
act, are lower than they otherwise would be with a high 
temperature of final delivery into the mains. Any heat 
which the air entering the mains may possess above the 
temperature of the atmosphere will obviously be lost during 
the long journey to the distributing pipes and motors. 
The less heat, consequently, that is generated during the 
process of compression, the more economical in power that 
process will be. Every means for keeping the air cool that 
can with advantage be used, such as jacketing the cylinders 
with cold water, injecting water into the cylinders as a fine 
spray, and in the case of compound compressors the adop- 
tion of inter-coolers, is consequently resorted to. The air 
finally delivered by the compressor must, if it has been 
subjected to spray injection, before it passes into the mains, 
be deprived of all particles of water suspended amongst it. 
This is done by passing it through reservoirs containing 
baffle plates, which separate out the water particles from 
the air by the action of the surface film on the little drops, 
and by gravity. 
Air Mains.—The pipe for conveying the compressed air 
from the source of power to the locality where it is to be 
distributed is preferably as smooth in its interior as possible, 
If large enough to be a riveted structure, the heads of the 
rivets inside should be countersunk, so as to offer no pro- 
jections to increase the resistance. The joints which con- 
nect together successive lengths of pipe, must be flexible to 
some extent, so as to allow of the pipe line yielding to 
lateral movements of the earth which surrounds it. They 
must at the same time be and remain so tight that the loss 
of air by leakage is inappreciable. 
These conflicting conditions are perfectly capable of a 
satisfactory simultaneous fulfilment. In the mains of Paris, 
