of a large loss
in the present method of compressing air. Here we have a simple engine,
compact and complete in itself, capable of high speed without injury,
constructed on the basis of our best steam engine practice, which
produces compressed air power at a loss of only six per cent.

Clearance is not taken into consideration in the foregoing figures, but
clearance is very much more of a _bete noir_ in theory than in practice.
The early designers, as shown in the "Dubois-Francois" illustrations,
Figs. 3 and 4, regarded clearance loss as a very serious matter. Even at
the present time some air compressor manufacturers admit water through
the inlet valves into the air cylinder, not so much for the purpose of
cooling as to fill up the clearance space. A long stroke involving
expensive construction is usually justified by the claim that a large
saving is effected by reduced clearance loss. Let us see what the effect
of this clearance is. Assuming that we have an air compressor which
shows an isothermal pressure line, there would be some loss of power due
to clearance space, because we would have a certain volume of air upon
which work was done and heat produced, that heat having been absorbed
and the air being retained in the cylinder and not serving any useful
purpose. But let us assume that we have a compressor which shows an
adiabatic pressure line. We now have the air in the clearance space
acting precisely as a spring, compressed at each stroke, retaining its
heat of compression, and giving it out against the air piston at the
point when the stroke is reversed. There is no loss of power in such a
case as this, but, on the contrary, the air spring is useful in
overcoming the inertia of the piston and moving parts. The best air
compressors give a result about midway between the isothermal and the
adiabatic, and the net loss of _power_ directly due to clearance is so
small as to be practically unworthy of consideration.

It must not be inferred from the preceding remarks that the designer of
an air compressor may neglect the question of clearance. On the
contrary, it is a very important consideration. If we assume a large
clearance space in the end of an air cylinder of a compressor which is
furnishing air at a high pressure, we may readily conceive that space to
be so large, and that pressure so high, that the entire volume of the
cylinder would be filled by the air from the clearance space alone, and
the compressor w