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.
In Fig. 29 we have a triple-expansion marine engine. Steam enters the high-pressure cylinder4 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°, 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