STIRLING, ROBERT (1790–1878)

Robert Stirling was born on October 25, 1790, in the parish of Methvin, Perthshire, Scotland. Stirling, the son of a farmer, received a classical education followed by studies in divinity at the universities of Edinburgh and Glasgow. The Church of Scotland, after examining his knowledge of Hebrew, Greek, divinity, and church history, licensed Stirling to preach the Gospel in March 1816. Shortly after, Stirling accepted the patronage of the Duke of Portland, who proposed Stirling as a suitable candidate for the vacant post of assistant minister at Kilmarnock. Stirling was ordained on September 19, 1816.

Just eight days after his ordination, Stirling applied for a patent for "Improvements for diminishing the consumption of fuel." He had evidently carried out his experiments while he was a student at Edinburgh, but why a student of divinity should have developed an interest in fuel economy, and gone to the expense of applying for a patent to protect his invention, is something of a mystery. Stirling's patent was in several parts. The first part was for a method of diminishing the quantity of fuel used in furnaces for melting glass, while still producing the high degree of heat required. The second was for a heat exchanger suitable for use in breweries and factories where economies could be made by transferring heat from one fluid or vapor to another. The third was for an engine for moving machinery.

Stirling's engine consisted of an externally heated, hot-air engine fitted with what he termed an "economiser." The economiser absorbed and released heat to and from the enclosed air, which was shifted alternately between hot and cold spaces, with the resulting pressure changes communicated to a power piston. Stirling was the first to design an engine with a working cycle that depended on recovering waste heat for efficient operation.

Stirling's patent was based on practical experiments and a working knowledge of radiant heat. Stirling may have been influenced by the works of John Leslie, Professor of Mathematics at Edinburgh, who in 1804 published the results of his investigations into the nature of radiant heat. The general layout of Stirling's engine embodies Leslie's experiments with spaced radiation screens to reduce the transmission of heat. In a second work published in 1813, Leslie argued that heat was transmitted as a series of vibrations, and if heat were applied to one end of a cylinder of metal, then a regular descending graduation of temperature would soon be established along the whole length. Stirling seems to have perceived that here was a process that might be made reversible. If heat were transmitted in a series of vibrating layers, and if the heat source were itself moved along the layers, heat could be discharged into bands of heat at decreasing temperatures. That is to say, as the heated mass touched each layer, the temperature of the heated mass would fall. If the mass of air were to be passed back over the bands of heat, then it would regain its former temperature. This was how Stirling described the workings of his economiser, although he took care to dispel any idea of perpetual motion by pointing out the losses.

Stirling's transfer to Kilmarnock did not put a stop to his experimenting. Here he meet Thomas Morton, an inventor and manufacturer who supplied him with a workshop. Morton also built telescopes and observatories and was influential in developing Stirling's interest in constructing optical and scientific instruments. In 1818 Stirling erected an engine of his own design to drain water from a local stone quarry. This engine performed its duty well until, as a result of carelessness of the engine-man, it over-heated and was rendered useless.

James Stirling, who was ten years younger than his brother Robert, initially studied for the church, taking a classical education at Edinburgh and Saint Andrews University. The pull of mechanics proved stronger than religion, and he took up an apprenticeship with Claud Girdwood and Co., of Glasgow, machine makers and iron founders. In 1824 he suggested his brother use compressed air for his engine rather than air at atmospheric pressure, and by 1827 James was influential enough to have built an experimental twenty horse-powerengine fitted with a sheet iron economiser, in which too much faith was placed, resulting in the engine failing for want of effective cooling. This engine embodied a number of improvements in the economiser, which were patented in 1827 in the joint names of Robert and James Stirling. James left Girdwood's in 1830 to become manager of the Dundee Foundry, Dundee.

At Dundee the brothers continued experimenting, directing their efforts into improving the efficiency of both cooling apparatus and economiser. The results of these experiments were patented in 1840 (the year Robert was awarded a Doctor of Divinity), and in 1843 a forty-five horsepower engine was set up to power the whole of the foundry, which it did until 1847. The trial engine successfully vindicated the Stirling brothers' claims for improved fuel efficiency, but ultimately failed as a practical engine because the available material—cast iron—could not withstand the high temperatures required for efficient operation. The development of the Stirling engine was effectively halted until improvements in metallurgy were made.

In 1837 James married Susan Hunter, the daughter of a Saint Andrews Professor; they had no children. James left Dundee in 1845 to become a successful consulting engineer in Edinburgh, where he died at the age of seventy-five. In 1819 Robert had married Jane Rankine, the daughter of a Kilmarnock merchant. They had two daughters and five sons. Robert had moved to the parish of Galston in 1823, and he remained a respected minister until his death at the age of eighty-eight, although some of his parishioners felt he devoted too much time to his mechanical pursuits.

Such was Robert Stirling's interest in science and engineering that of his sons, four became leading railway engineers in Britain and South America, and one became a church minister. His son-in-law became the general manager of an iron works. Stirling never sought to benefit from his invention; indeed, he did little to prevent infringements of his patents. Stirling's 1816 patent foresaw all possible applications of what came to be called the regenerative process, a term coined by John Ericsson, who never accepted the priority of Stirling's patent. However, Ernst Warner von Siemens, who profitably developed the regenerative furnace for smelting metals, did acknowledge Stirling as the originator.

Robert Sier

See also: Ericsson, John; Siemens, Ernst Warner von; Stirling Engines.

BIBLIOGRAPHY

Sier. R. (1987). A History of Hot Air and Caloric Engines. London: Argus Books.

Sier. R. (1995). Rev. Robert Stirling D.D.Chelmsford: L. A. Mair.