James Watt’s invention of the condensing steam engine in 1769, which was conceived while he was mending the very basic Newcomen steam engine, was a crucial element in the advance of the industrial revolution and the railways and ships that were central to it. Towards the end of his life he was involved in various patent disputes, and in the following letter to his friend John Robison, railing against one such battle, he neatly summarizes the steps that led to his groundbreaking invention.
Heathfield, Birmingham, 24 October 1796
My Dear Sir …
I have been obliged to trouble many of my friends with these abominable law affairs but have hitherto, have only called those who lived near London, but now these scoundrels, – the Hornblowers and others have leagued against us we must call all who are willing to help us …
You will see from the papers sent you the objections and consequently the proper answers, I propose to send you my own general reply for your government, as soon as copy can be made. – The point is to establish that I was the inventor that the invention was perfect as to the saving steam and fuel at the time of the patent 1769, and that the specification is sufficient to enable a Mechanick understanding Newcomens Engine to have constructed one with these properties.
I did not invent this method piece meal but all at once in a few hours in 1765 I believe. The first step was the idea from the elastic nature of steam of condensing in a seperate [sic] vessel, 2d the getting out the water by a long pipe and the air by a pump, 3d that the pump would extract the water also 4th that grease might be used in place of water to keep the piston tight, 5th that Steam might be employed to press upon the piston in place of air 6 to keep the Cylinder warm.
The next day I set about it. The boiler was ready, I took a large syringe of Tom Hamiltons 2 inches diar [diameter] and a foot long that was the Cylinder. I made two tin ends to it with a pipe to convey steam to both of them. I made a tin Condenser consisting of a pump about an inch diar, and two small pipes about 10 inches long and 1/8 diar immersed in a small round cistron, which I still have. I placed the Cylr [cylinder] inverted tied a weight to the piston rod, blew out the air and condensed water through the piston rod, which was hollow, and when I judged the Cylr filled with steam, I drew up the piston of the pumps and the weight immediately followed, to my great Joy, all this was done in a day or two after I had contrived it …