It is evident that an air compressor which has the steam cylinder and the air cylinder on a single straight rod will apply the power in the most direct manner, and will involve the simplest mechanics in the construction of its parts.  It is evident, however, that this straight line, or direct construction, results in an engine which has the greatest power at a time when there is no work to perform.  At the beginning of the stroke steam at the boiler pressure is admitted behind the piston, and, as the air piston at that time is also at the initial point in the stroke, it has only free air against it.  The two pistons move simultaneously, and the resistance in the air cylinder rapidly increases as the air is compressed.  To get economical results it is, of course, necessary to cut off in the steam cylinder, so that at the end of the stroke, when the steam pressure is low, as indicated by the dotted line (Fig. 5), the air pressure is high, as similarly indicated.  The early direct-acting compressor used steam at full pressure throughout the stroke.  The Westinghouse pump, applied to locomotives, is built on this principle, and those who have observed it work have perhaps noticed that its speed of stroke is not uniform, but that it moves rapidly at the beginning, gradually reducing its speed, and seems to labor, until the direction of stroke is reversed.  This construction is admitted to be wasteful, but in some cases, notably that of the Westinghouse pump, economy in steam consumption is sacrificed to lightness and economy of space.

[Illustration:  FIG. 5.]

Many efforts were made to equalize the power and resistance by constructing the air compressor on the crank shaft principle, putting the cranks at various angles, and by angular positions of steam and air cylinders.  Several types are shown in Fig. 6.

[Illustration:  FIG. 6.]

Angular positions of the cylinder involve expensive construction and unsteadiness.  Experience has conclusively proved that it does not pay to build air compressors with vertical cylinders, and moreover we have found out that there is nothing in the apparent difficulty in equalizing the strains in a direct-acting engine.  It is simply necessary to add enough weight to the moving parts, that is, to the piston, piston rod, fly wheel, etc., to cut off early in the stroke and secure rotative speed with the most economical results and with the cheapest construction.  It is obvious that the theoretically perfect air compressor is a direct-acting one with a conical air cylinder, the base of the cone being nearest the steam cylinder.  This, from a practical point of view, is impossible.  Mr. Hill, in referring to the fallacious tendencies of pneumatic engineers to equalize power and resistance in air compressors, says:  “The ingenuity of mechanics has been taxed and a great variety of devices have been employed.  It is usual to build on the pattern of presses which do their work in a