The growth of the demand for economy in blowing engines, previously described, obviously required the introduction of compounding, to which at first sight the long cross-head type of engine did not adapt itself well.

The late John Fritz solved the problem by putting the high and low pressure cylinder of the compound side by side over the shaft in the position previously occupied by the single cylinder, and connected both piston rods to the long cross head, the position and parallelism of this being rigidly controlled by the connecting rods at its outer ends, by which the continually varying differences in pressures in the two cylinders were absorbed, apparently without injurious racking stresses on the engine, A single air cylinder was used, as with the simple type of engine, its piston rod being connected to the center of the long cross head as before.

This type of engine, however, never received general favor, and, consequently, other types were developed which adapted themselves to compounding. The principal types are the horizontal cross-compound with tandem air cylinders, the vertical cross-compound with tandem air cylinders, known as the steeple type, the cross-compound horizontal vertical engine having horizontal steam cylinders working on two cranks at right angles, with the air cylinders vertically over these cranks and driven by them, the quarter crank vertical engine with steam and air sides entirely separated except for the crank shaft which connects them, and the disconnected compounds.

Fig. 90. Air heads equipped with "Mesta" automatic plate valves (Iverson patent).

Before we can discuss these different types of engines intelligently the subjects of the inertia of the reciprocating parts and engine friction must be briefly treated. The subject, as applied to the steam engine, will be found in any good treatise on that subject, but the power-generating steam engine is very different from any type of blowing engine in which the air and steam pistons are connected to the same rod, for the reason that in the power engine all work generated in the cylinder has to pass through moving parts, the cross-head pin, crank pin and main bearing, into the crank shaft, from which the useful effect is delivered. In the blowing engine on the other hand, where the steam and air pistons are rigidly connected to the same rods, the energy generated in the steam cylinder is transmitted only through a rigid member directly into the air cylinder, except, as has been pointed out, that most of the work is generated in the steam cylinder in the early part of its stroke, and the principal utilization in the air cylinders occurs in the latter part of the stroke.

It is obvious that the excess above that immediately utilized in the air cylinder must be stored either in the fly-wheel or in the reciprocating parts and given out again to make good the deficiency in the latter part of the stroke. The transmission of these excesses and deficiencies through running parts affects seriously the friction of the engine, which is important not only on account of the power so lost, but because the power lost represents destructive energy applied to the moving parts of the engine itself, and for economy of maintenance, even if not for economy of power, this should be kept as low as possible.

It is probably true to say that the friction of a well-built reciprocating steam engine is about 10 per cent. of its indicated horse-power at its rated load, while when running idle, but up to speed, the friction is about one-half of this, that is, about 5 per cent. of its rated power.

It is obvious, therefore, that if we can take part of the energy developed in the steam cylinder and apply it directly without its having to pass through the running parts to the fly-wheel, we shall make a considerable saving in friction, and, consequently, in wear.