The horizontal-vertical, shown in Fig. 96, presents almost the same conditions, though these are offset by the fact that the running balance of these engines, if properly made, is excellent, and therefore their steadiness on their foundations very great.

On the other hand, the horizontal-vertical requires as much height and much greater foundation than the quarter-crank vertical.

Many years ago the late P. L. Weimer, realizing that the energy of the steam was mainly developed in the early part of the stroke, and absorbed by the air mainly in the latter part of its stroke, built what are now known as horizontal-vertical engines, with horizontal steam cylinders and vertical air cylinders, both working on the same crank pin. With this arrangement the earlier part of the steam stroke obviously takes place during the latter part of the air stroke and vice versa. Consequently, a certain equalization of the work absorbed and expended from instant to instant is made, which justified in part the use of this type when speeds were low and inertia effects negligible.

In later years these engines have been built by other builders in very large sizes, and are favorably considered by some operator, although the inertia effect with moderate speeds makes this design wholly unnecessary. In these engines, also, it is obvious that the total work generated in the steam cylinder must be transmitted to the crank pin with a friction loss of some 10 per cent. and then transmitted back from the crank pin through another set of running parts to the air cylinder. It is reasonable to assume that here also there is a loss in friction of 10 per cent., making the mechanical efficiency of this engine also, as a whole, only about 81 per cent.

I have unfortunately no figures giving the mechanical efficiency of these engines in service, but there is no reason to expect that it is much higher than the figure given. On the other hand, with the direct-connected steam and air pistons of the horizontal tandem and a proper proportioning of the cylinder diameters, speed, etc., it is obvious that only about 20 or 25 per cent. of the total energy need be transmitted through the running parts, and that the friction of the engine accordingly should be only 6 or 7 per cent. instead of 19, as in the other case.

In confirmation of this, which may seem an extreme statement, tests have been made on Nordberg compressors designed to make use of this principle, which had a mechanical efficiency of 95 per cent. at full load.

There is, however, one feature of the horizontal-vertical engine which deserves attention. It is evident from what we have said above that the inertia of the reciprocating parts can be utilized to reduce the stresses in the running parts very greatly. Nevertheless the inertia stresses due to these heavy parts which move at high velocities alternately in opposite directions, exist in the machine as a whole and tend to shift it back and forth on its foundation in the case of the horizontal engine without counterweight, or to pull the crank shaft up and down with an equal force if full counterweight be used, these conditions being reversed in the case of the vertical engine. But when two sets of reciprocating parts at right angles work on the same crank, the inertia effect of one set is horizontal and that of the other is vertical, these components combine into a single constant stress in a direction exactly opposite to that of the crank, so that by counterweighting completely one set of the reciprocating parts, the horizontal-vertical engine runs in almost perfect balance.

Where foundations are bad and engines must be set upon piles driven through mud to the bedrock, these considerations are of much importance, and this type of engine is the best adapted of any to prevent serious shaking of the engine and its foundation or even the whole surrounding country for a distance of many hundred feet, as has happened at one large plant where the horizontal blowing engines were supported on piles.