compressors of the dry system one-half of this loss is saved by water
jacket absorption, so that we are left with about 11 per cent., which
the slow moving compressor seeks to erase. We are quite safe in saying
that the element of _time alone_ in the stroke of an air compressor
could not possibly effect a saving of more than half of this, or 5½ per
cent. Now, in order to get this 5½ per cent. saving, we reduce the speed
of an air-compressing engine from 350 feet per minute to 100 feet per
minute. We must, therefore, in one case have a piston area _three and
one-half_ times that of the other in order to get the _same capacity of
air_, and in doing this we build an engine of enormous proportions with
heavy moving parts. We load it down with a large mass of water, which it
must move back and forth during its work, and thus we produce a
percentage of friction loss alone equal to twice or even three times the
5½ per cent. heat loss which is responsible for all this expense in
first cost and in maintenance, but which really is not saved after all
unless water injection in the form of spray also forms a part of the
system.

It is obvious that cost of construction and maintenance have much to do
with the commercial value of an air compressor. The hydraulic piston
machine not only costs a great deal more in proportion to the power it
produces, but it costs more to maintain it, and it costs more to run it.
It is not an uncommon thing to hear engineers speak of the hydraulic
piston compressor as the "most economical" machine for the purpose, but
that it is so "expensive" and takes up so much room, and requires such
expensive foundations that, unless persons are "willing to spend so much
money," they had better take the next best thing, a high speed machine.
We hear of "magnificent air-compressing engines, the largest in the
country," and pilgrimages are made to see these artificial wonders when,
not unlike the old pyramids, they represent a pile of inert matter--a