Theoretically, by increasing the _lap_ and cutting off the steam earlier and earlier in the stroke, we should economize our power more and more. But in practice a great difficulty is met with--namely, that _as the steam expands its temperature falls_. If the cut-off occurs early, say at one-third stroke, the great expansion will reduce the temperature of the metal walls of the cylinder to such an extent, that when the next spirt of steam enters from the other end a considerable proportion of the steam's energy will be lost by cooling. In such a case, the difference in temperature between admitted steam and exhausted steam is too great for economy. Yet we want to utilize as much energy as possible. How are we to do it? COMPOUND ENGINES. In the year 1853, John Elder, founder of the shipping firm of Elder and Co., Glasgow, introduced the _compound_ engine for use on ships. The steam, when exhausted from the high-pressure cylinder, passed into another cylinder of equal stroke but larger diameter, where the expansion continued. In modern engines the expansion is extended to three and even four stages, according to the boiler pressure; for it is a rule that the higher the initial pressure is, the larger is the number of stages of expansion consistent with economical working. [Illustration: FIG. 29.--Sketch of the arrangement of a triple-expansion marine engine. No valve gear or supports, etc., shown.] In Fig. 29 we have a triple-expansion marine engine. Steam enters the high-pressure cylinder[4] at, say, 200 lbs. per square inch. It exhausts at 75 lbs. into the large pipe 2, and passes to the intermediate cylinder, whence it is exhausted at 25 lbs. or so through pipe 3 to the low-pressure cylinder. Finally, it is ejected at about 8 lbs. per square inch to the condenser, and is suddenly converted into water; an act which produces a vacuum, and diminishes the back-pressure of the exhaust from cylinder C. In fact, the condenser exerts a _sucking_ power on the exhaust side of C's piston. ARRANGEMENT OF EXPANSION ENGINES. In the illustration the cranks are set at angles of 120 deg., or a third of a circle, so that one or other is always at or near the position of maximum turning power. Where only two stages are used the cylinders are often arranged _tandem_, both pistons having a common piston rod and crank. In order to get a constant turning movement they must be mounted separately, and work cranks set at right angles to one another. COMPOUND LOCOMOTIVES.