This impressive model, shown at Harrogate, is by S Naisbitt. It is a marine triple expansion steam engine.

Compound marine steam engines were extensively produced from the end of the 19th century up until the end of World War II. Until the development of compound engines, steam engines used the steam only once before being recycled back to the boiler. A compound engine recycles the steam into one or more larger, lower-pressure, cylinders to utilize more of its heat energy. Compound engines could be configured either  to increase a ship's economy or its speed.

A compound engine refers to a steam engine with any number of different-pressure cylinders, usually through two stages, i.e. those which operate cylinders at only two different pressures (or ‘double-expansion’ engines).

The triple-expansion engine is a compound engine

that expands the steam in three stages. It has three

cylinders operating at different pressures.

A quadruple-expansion engine expands the

steam in four stages, and so on. The first successful

commercial use was an engine built at Govan

by Alexander C. Kirk for the SS Aberdeen in 1881.

Multiple-expansion engine manufacture continued

well into the 20th century. All 2,700 Liberty ships

built by the United States during World War II were

powered by triple-expansion engines, as

capacity to build marine steam turbines was

focussed on the building of warships.