Note 215 — Chapter V

The compound engine has two cylinders, one frequently double the diameter of the other. Steam at a high pressure is admitted from the boiler into the smaller cylinder, and, after it has driven the piston up or down, it is allowed to pass into the larger cylinder when, by its expansive property, it assists in driving the larger piston down or up. When it has exerted its full expansive effect, it is then in a condition to be condensed; and thus a compound engine combines the advantages of both a high pressure and low pressure or condensing engines. Compound engines are sometimes called “high and low pressure” engines, and there are a great many modifications of them, their principle however, being always the same.