side of a large body of air is exposed to a water surface, and as water
is a bad conductor, the result is that a thin film of water gets hot in
the early stage of the stroke and little or no cooling takes place
thereafter. The compressed air is doubtless cooled before it gets even
as far as the receiver, because so much water is tumbled over into the
pipes with it, but to produce economical results the cooling should take
place _during compression_.

Water and cast iron have about the same relative capacity for heat at
equal volumes. In this water piston compressor we have only one cooling
surface, which soon gets hot, while with a dry compressor, with water
jacketed cylinders and heads, there are several cold metallic surfaces
exposed on one side to the heat of compression, and on the other to a
moving body of cold water.

But the water piston fraternity promptly brings forward the question of
speed. They say that, admitting that the cooling surfaces are equal, we
have in one case _more time_ to absorb the heat than in the other. This
is true, and here we come to an important class division in air
compressing machinery--_high speed and short stroke_ as against _slow
speed and long stroke_. Hydraulic piston compressors are subject to the
laws that govern piston pumps, and are, therefore, limited to a piston
speed of about 100 feet per minute. It is quite out of the question to
run them at much higher speed than this without shock to the engine and
fluctuations of air pressure due to agitation of the water piston. The
quantity of heat produced, that is, the degree of temperature reached,
depends entirely upon the conditions in the air itself, as to density,
temperature and moisture, and is entirely independent of speed. We have
seen that it is possible to lose 21.3 per cent. of work when compressing
air to five atmospheres without any cooling arrangements. With the best