In 1816 the library of the Patent Office has been the recipient of the original specification in the handwriting of the inventor, the Rev. Robert Stirling, of the Heat "Regenerator" and the Stirling Air Engine (Letters Patent No. 4081 of 1816).
For some reason, not definitely ascertained, this specification, though duly signed, attested and stamped, was not enrolled. Hence, it does not appear in the Official Blue-book series, and has always been treated as a lapsed application.
It is known that copies of the Scotch specification were produced in evidence in the celebrated trials of the Neilson Hot-blast patent. It is, we believe, established that the first application of the regenerative principle to iron smelting by James Baird in 1825 was due to the suggestion of Robert Stirling, who, it is stated, had included "iron smelting furnace" in the original draft of his specification, but had struck out the words, and gave only one example, selecting a glass furnace for the purpose.
This omission probably saved Neilson’s patent, while it accounts for the fact that Stirling was not called to give evidence at the Edinburgh trial in spite of the pressure put upon him by the defendants.
It should be observed that the term "regenerative furnace" was not coined by Stirling, who elsewhere stated that he preferred to describe his invention as an "Economiser", but the term coined by Ericsson was so firmly rooted in the language in Siemens' time that the latter accepted it under protest (Proc. Inst. C.E., vol. xii., 1852-3). Further, it is worthy of note that nearly all the applications of the regenerative principle in heating and cooling were foreseen by Stirling, and indicated by him in his specification.
The principle of the Stirling Air Engine differs from that of Sir George Cayley (1807), in which the air is forced through the furnace and exhausted, whereas in Stirling’s engine the air works in a closed circuit. It was to it that the inventor devoted most of his attention.
A two horse-power engine, built in 1818 for pumping water at an Ayrshire quarry, continued to work for some time, until a careless attendant allowed the heater to become overheated. This experiment proved to the inventor that, owing to the low working pressure obtainable, the engine could only be adapted to small powers for which there was at that time no demand.
His younger brother James Stirling suggested, in 1824, using compressed air, and in 1843 built an engine of 45 brake horse-power, which successfully drove the machinery at the Dundee Foundry for several years, thus demonstrating the adaptability of the principle to higher powers.
It may be of interest to note here that Robert Stirling was born at Cloag, Methvin, Perthshire, in 1790, and entered the Church. At the date of his invention, 1816, he was twenty-six years of age, and had just been ordained to his first parish. He was greatly esteemed as a minister, and was a noted classical scholar, as well as a scientist, and before he died in 1878, was the Father of the Church of Scotland. His grandfather, Michael Stirling, of Glassingal, Dumblane, invented the first rotary thrashing machine in 1756 (Encyclopaedia Britanica). His brother James was a celebrated Civil Engineer in Edinburgh, and four of his sons were engineers, who all made their mark in the engineering world. Two of them, Patrick and James, are well known in this country to locomotive engineers of the present generation.
Robert Stirling patent of 1816, which entire writing is given here after, has been published in extenso for the first time by the The Engineer of Dec. 1917 at the occasion of the centenary of the Stirling Engine.
The first part of the patent relates to the description of Stirling's Economiser (which has become the regenarator). Robert Stirling gives not only a detailled description but explains carefully the purpose of each items.
The second part is not exclusively about the hot air engine. Stirling writes "I shall now describe several of its numerous and useful applications". One of these being the Stirling hot air engine.