are used with questionable engineering skill and very poor results."

[Illustration: FIG. 7.]

Fig. 7 illustrates the theory of Duplex Air Compressors. The hydraulic
piston or plunger compressor is largely used in Germany and elsewhere on
the Continent of Europe, but the duplex may be said to be the standard
type of European compressor at the present time. It is also largely used
in this country. Fig. 7 shows the four cylinders of a duplex compressor
in two positions of the stroke. It will be observed that each steam
cylinder has an air cylinder connected directly to the tail rod of its
piston, so that it is a direct-acting machine, except in that the
strains are transmitted through a single fly wheel, which is attached to
a crank shaft connecting the engines. In other words, a duplex air
compressor would be identical with a duplex steam engine were it not for
the fact that air cylinders are connected to the steam piston rods. The
result is, as shown in Fig. 7, that, at that point of the stroke
indicated in the top section, the upper right hand steam cylinder,
having steam at full pressure behind its piston, is doing work through
the angle of the crank shaft upon the air in the lower left hand
cylinder. At this point of the stroke the opposite steam cylinder has a
reduced steam pressure and is doing little or no work, because the
opposite air cylinder is beginning its stroke. Referring now to the
lower section, it will be seen that the conditions are reversed. One
crank has turned the center, and that piston which in the upper section
was doing the greatest work is now doing little or nothing, while the
labor of the engine has been transferred to those cylinders which a
moment before had been doing no work.

There are some advantages in this duplex construction, and some