stroke, and to equalize the power and resistance.  In other words, a fly wheel is not a source of power, and in many cases it is only a means by which we accomplish rotative speed.  It takes power to move matter, and assuming that other conditions are equal, every engine that carries a fly wheel that is larger than is necessary consumes a certain number of foot pounds in turning so much metal around through space.  Were it possible to cut off at the same point and rotate as positively without a fly wheel, it would be done away with entirely.  Some straight line air compressors are so constructed that the momentum of the piston and other moving parts is nearly sufficient to equalize the strains without a fly wheel; but the fly wheel is there because it insures a definite length of stroke, and because it enables us to operate eccentrics and to regulate the speed of the engine uniformly.

Objections to the duplex construction are:  The strains are indirect, angular and intermittent.  It is necessary therefore to largely increase the strength of parts; to add a crank shaft of large diameter with enormous bearings, and to build expensive and very secure foundations.  Should the foundations settle at any point, excessive strains will be brought upon the bearings, resulting in friction and liability to breakage.  A steam engine meets with a resistance on its crank shaft that is uniform throughout the stroke; while an air compressor is subject to a heavy maximum strain at the end of the stroke, hence the importance of direct straight line connection between power and resistance.

[Illustration:  FIG. 8.]

The friction loss on a duplex compressor seldom gets lower than 15 per cent., while straight line compressors show as low a loss as 5 per cent.  Fig. 8 illustrates the Rand Duplex Air Compressor, a machine largely used in America, especially in the Lake Superior iron mines.  Fig. 9 illustrates a Duplex Compound Condensing Corliss Air Compressor built by the Ingersoll-Sergeant Drill Company.  This is a compressor made of the best type of Corliss engine, with air cylinders connected to the tail rods of the steam cylinders.  One of these machines, of about 400 horse power capacity, is now at work furnishing compressed air power for the Brightwood Street Railway in Washington, D.C.  Fig. 10 illustrates the Norwalk direct-acting straight line air compressor, with compound air cylinder.  The chief purpose of compounding is to reduce the maximum strain.  This construction also adds to isothermal economy.  The large cylinder to the left determines the capacity of the compressor, the air being compressed first to a low pressure (ordinarily about 30 pounds per square inch), afterward passing through an intercooler, by which its temperature is reduced, and then it is compressed still higher, even to 5,000 pounds per square inch if desired.  The terminal strain, which is so severe in air compressors, is here considerably reduced, as in this case it is only equal to the area of the initial air piston multiplied by its low air pressure.