few inches of the end of the stroke and employ heavy fly wheels, extra strong connections, and prodigious bed plates.  Counterpoise weights are also attached to such machines; the steam is allowed to follow full stroke, steam cylinders are placed at awkward angles to the air-compressing cylinders and the motion conveyed through yokes, toggles, levers; and many joints and other devices are used, many of which are entire failures, while some 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 disadvantages.  The crank shafts being set quartering, as is the usual construction, the engine may be run at low speed without getting on the center.  Each half being complete in itself, it is possible to detach the one when only half the capacity is required.  The power and resistance being equalized through opposite cylinders, large fly wheels are not necessary.  Strange to say, the American practice seems to be to attach enormous fly wheels to duplex air compressors.  It is difficult to justify this apparently useless expense in view of the facts shown in Fig. 7.  A fly wheel does not furnish power, nor does it add to the economy of an engine except in so far as it enables it to cut off early in the