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The paper entitled "The Origins of James Watt's Perfect Engine" by Richard Hills, is published in the Transactions of the Newcomen Society, 1996-97 Vol 68.
In this paper, Dr Hills describes the developments in the 10 years between James Watt's first idea for a "perfect engine" and its fruition. In these excerpts, some of Watt's early thinking on this subject is laid before us.
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| The model Newcomen engine © copyright |
Watt had often been asked to repair apparatus before, and his friend Robison, with whom he had experimented earlier on model steam engines, commented:
this model was, at first, a fine plaything to Mr Watt, and to myself, now a constant visitor at the workshop. But, like everything which came into his hands, it soon became an object of most serious study.
"...Watt went far beyond just making (the model) work but sought to understand the basic principles. From these he conceived his idea of the perfect steam engine. In the first surviving letter which Watt sent in August 1765 to Dr. John Roebuck, he said, 'On the faith of this, I have set about a larger and more perfect model, having now little doubt of its performing to satisfaction’. A couple of weeks later, a similar expression is used in another letter to Dr Lind:
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| The earliest sketch we have of Watt's "perfect engine", from a letter to Dr Roebuck, 1765. © copyright. |
I have tried my small model of my perfect engine which hitherto answers expectation. and gives great, I may say greatest, hopes of success (for certainty could not be called hope). in greater model now far advanced; in short, I expect almost totally to prevent waste of steam and consequently to bring the machine to its ultimatum.
This letter to Lind gives us a clue about the perfect engine, that it would not waste any steam. This is confirmed by what happened when Robison entered Watt's room unexpectedly and found him playing with a 'little tin cistern’. Watt said briskly. 'You need not fash yourself any more about that, man; I have now made an engine that shall not waste a particle of steam. It shall all be boiling hot: - aye, and hot water injected if I please'. Robison had seen Watt's first condenser."
"...Watt must have pondered upon the dilemma that faced him throughout the winter of 1764-65. He realised that to make a perfect steam-engine, it was necessary that the cylinder should be always as hot as the steam which entered it, and that the steam should be cooled down below 100 degrees in order to exert its full powers. The gain by such construction would be double: first, no steam would be condensed on entering the cylinder; and secondly, the power exerted would be greater as the steam was more cooled. The postulata however, seemed to him incompatible, and he continued to grope in the dark.
The solution came to him on that famous walk across the Green of Glasgow on a Sabbath afternoon in the spring of 1765:
I was thinking upon the engine ... when the idea came into my mind that as steam was an all elastic body it would rush into a vacuum and if a communication was made between the cylinder and an exhausted vessel. it would rush into it, and might be there condensed without cooling the cylinder ... I had not walked further than the Golf-house when the whole thing was arranged in my mind.
At last Watt realised how he could keep his steam cylinder hot so that no steam would be condensed on entering it and then condense the steam in a separate condenser which could be kept cold and so create a perfect vacuum. But it would take him a very long time to make a practical working engine."
The complete text of this paper can be purchased on line from our archive.
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