Drawing of condenser and air pump from Watt's letter to Dr. Small on 22 February 1767

According to Watt's own account, the concept of a separate condenser first entered his mind in the spring of 1765. It had taken him two years after he first received a Newcomen's engine model [1], and a number of experiments, to come up with a solution to the issues he had identified. Watt immediately began to construct a model. Finally, in August 1765 his experiments with the model gave him confidence that he was on the right track [2]. When he was surveying various canal projects for a living, Watt must have put his experiments on hold. At the end of 1767, he appears to have started working on his invention again [3]. Around the same time Watt was introduced by Dr. Black to Dr. John Roebuck, who had been practising as a doctor in Birmingham, but at the age of forty-seven had become interested in chemical research and the manufacture of iron in Scottland. [4]. Roebuck, quite well off at the time, had sunk coal mines but his pits became flooded. The atmospheric engine he used to unwater his pit was not sufficiently powerful to do the job.

From there it took James Watt a lot of time to start writing and to perfect his patent specification. In 1769 he wrote to Dr. Small, a friend, 'as I have been informed that some patents have been defeated because the specification was not clear enough to enable other people to execute the scheme I have added descriptions of the machines with drawings. On the advice of his friend Dr. Small, Watt ultimately decided to omit the drawings [4]. Was Dr. Small concerned that the patent claim might be restricted as a result of the drawings?

However, the drawing that James Watt had created for his patent applications could be found in his personal papers and is shown below.

New Invented Method of Lessening The Consumption of Steam And Fuel in Fire Engines

Watt's steam engine with condenser. Illustration from Beck, Ludwig: Die Geschichte des Eisens. Bd. 3: Das XVIII. Jahrhundert. Braunschweig, 1897; colourized by Günther Schmalz (C) 2023.

James Watt was granted his first patent N° 913 on 5 January 1979. He was given four months to file a specification. After having submitted the specification on 25 April 1769 his patent was "inrolled" on 29 April 1769.

When James Watt applied for a patent, the steam engine with a condenser existed only as a modell. Unfortunately, his partner John Roebuck, who had agreed to finance the production of a steam engine went bankrupt. James Watt argued that he had employed many years a considerable part of his  fortune in making experiments upon steam and steam engines, but upon account  of the many difficulties which always arise in the execution of such large and complex machines he could not complete his invention before the end of the year 1774. The patent granted to him in 1769 therefore may elapse before he can receive an advantage adequate to his labour, time and expenses. In 1775 James Watt submitted this petition to King George III to extend the term of his patent to carry out his invention into the complete execution which he wishes, and so as to render the same of the highest utility to the public. The English parliament granted James Watt an extension of 25 years.

Only with this extension of the term Watt's new partner Matthew Boulton accepted to compensate John Roebuck for his expenses in order to acquire Roebuck's share of the patent. As with John Roebuck, Watt and Boulton agreed on a 1:2 profit sharing. 

Articles of James Watt´s patent

James Watt's patent did not have specific claims, but the specification listed as articles what he was seeking protection for. The articles of the extension are the same as the original filed patent, apart that the "disclaimer" regarding article 4 was included at the end of article 4.

My method of lessening the consumption of steam, and consequently fuel, in fire engines, consists of the following principles:

  1. That vessel in which the powers of steam are to employed to work the engine, which is called the cylinder in common fire engines, and which I call the steam vessel, must during the whole time the engine is at work be kept as hot as the steam that enters it,

    1. by enclosing it in a case of wood, or any other materials that transmit heat slowly;
    2. by surrounding it with steam or hotter bodies;
    3. by suffering neither water or any other substance colder than the steam to enter or touch it during that time.

  2. In engines, that are to worked wholly or partially by condensation of steam, the steam is to be condensed in vessels distinct from the steam vessels or cylinders, although occasionally communicating with them; these vessels I call condensers, and whilst the engines are working, these condensers ought at least to be kept as cold as the air in the neighbourhood of the engines, by application of water or other cold bodies.

  3. Whatever air or other elastic vapour is not condensed by the cold of the condenser, and may impede the working of the engine, is to be drawn out of the steam vessels or condensers by means of pumps wrought by the engines themselves or otherwise.

  4. I intend in many cases  to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner as the pressure of the atmosphere is now employed in common fire engines; in cases where cold water cannot be had in plenty, the engines may be wrought by this force of steam only, by discharging the steam into the open air after it has done its office [which fourth article the said James Watt declares in a note affixed to the Specification of said engine should not be understood to extend to any engine where water to be raised enters the steam vessel itself, or any vessel having an open communication with it]. on horizontal axles, like the wheels of of a water mill; within placed a number of valves that suffer any body to go round the channel in one direction only; in these steam vessels are placed a number of valves so fitted to them as entirely fill up a part or portion of their channels, yet rendered capable of moving freely in them by the means herein after mentioned or specified. When the steam is admitted in these engines between these weights and the valves, it acts equally on both so as to raise the weights to one side of the wheel, and by the reaction on the valves successively to give a circular motion to the wheel, the valves opening in the direction in which the weights are pressed, but not in the contrary; as the steam vessels moves round, it is supplied with steam from the boiler, and that which has performed its office may either be discharged by means of condensers or into the open air.

  5. I intend in some cases to apply a degree of cold not capable of reducing the steam to water, but contracting it considerably, so that the engines shall be worked by the alternate expansion and contraction of the steam.

  6. Instead of using water to render the piston or to her parts of the engines air and steam-tight, I employ oils, was rosinous bodies, fat of animals, quicksilver, and other metals in their fluid state.
Drawing prepared by James Watt for his 1769 patent application. Retrieved by H.W. Dickenson & R. Jenkins in the Doldowlod Papers.

In this drawing, the air pump C empties the warm water from the condenser B into the container with the cooling water. The air pump C is operated by a rod from a horse head of the beam. Later illustrations reveal that the warm water is collected in a distinct basin known as the hot well. By replacing the water in the boiler with water from the hot well improved slightly the energy balance of the steam engine. A valve at the condenser's top end prevents water and air from the condenser's interior from being drawn back into the large cylinder A. Later installations will move this valve to the connection between the air pump and cylinder, allowing the cylinder and condenser to communicate without any obstruction.

Another valve between the boiler and the large cylinder A allows for interrupting the flow of steam from the boiler into the part of the cylinder below the piston as well as to the part of the cylinder above the piston. With Newcomb engines, water was supplied to the top side of the piston to soak it and reduce leakage. Watt applied instead steam at atmospheric pressure, which prevented leaks and heat losses simultaneously. 

We also see on the top of the drawing a "steam wheel" that, by use of liquid mercury was intended to produce rotary action. However, the practical difficulties of this construction were never solved and this idea was eventually not included in any of Watt's patent applications.

Over years preceding the patent application Watt had performed various experiments to optimize the Newcomen steam engine. The most important aspect is of course to move the condensation from the steam vessel to a an especially dedicated device, the condenser. The condenser was submerged in a basin with cold water. In the beginning the transfer of  steam from the steam vessel to the condenser was blocked by a valve which was only opened for the condensing process. However, Watt found out that this added little to the performance and abolished this valve. It's likely that he was able to do without this valve because, in the absence of a pump stroke sucking steam from the working cylinder into the condenser, the steam extending into the condenser did condense so slowly that it did not create enough underpressure to suck in sizable amounts of steam from the working cylinder.

Similarly, in the beginning, the inside volume of the condenser was flooded each time with new cold water. This water had to be pumped out with each stroke, adding additional work to the energy balance. Watt experimented with a variety of condenser shapes submerged in cooling water but without any water inside. The condensation took place excessively slowly. In the end, Watt drilled a hole in the condenser to let the cold water spray into it. He was satisfied that the condensation occurred instantly. Watt ultimately employed the same method as Newcomen, with the crucial distinction that he sprayed the cold water into a different vessel that did not require reheating. By only evacuating the injected water and the condensed water—which was less than the total volume of the condenser—the air pump consumed  less power. [4]

In the beginning the injection of the cold water was turned on and off with each stroke. Some engines were operating with a continuous stream of condensing water by 1791, and this became the standard practice. [H.W. Dickenson, page 212].

When calculating the performance of Watt's steam engines it was assumed that the condenser is supplied with cold condensing water, at a temperature of not more than 50 degrees of Fahrenheit's thermometer (10°C). The temperature of the hot water which is discharged by the air-pump, will be about 100 degrees of Fahrenheit (38°C) [5].

The patent application was based on the experiences made with a model engine. For that reason, the patent drawing, which was never filed, shows only a hand operated regulator, as the valve that cuts off the steam flowing from the boiler to the cylinder is called. Following the patent application, Watt built an engine of real size on Roebuck's premises in Kinneil, Scotland. The model confirmed that the external condensor is improving the consumption of steam. Nevertheless, with a real-size engine, additional improvements—like the size of the air pump and sealing the piston —arose. Even though Roebuck had paid for the patent application and permitted Watt to build an engine on his land, Watt was forced at some point to stop working on the steam engine and return to his previous job as a canal surveyor.

When John Roebuck became bankrupt in March 1773, he owed Matthew Boulton a considerable sum of money. Boulton, a manufacturer of metal products in Birmingham, negotiated a settlement with Roebuck in which Roebuck consented to give Boulton his two-thirds interest in Watt's patent. Consequently, the steam engine from Kinneil was disassembled and rebuilt in Boulton's Soho Factory. Watt didn't feel ready to sell a steam engine to a customer just yet. For the next two years Watt further improved his steam engine when finally in June 1775 he accepted to design two large engines for a customer, which were erected in March 1776.


[1] The model actually had no mechanical defect. Whoever built the model had reduced the dimensions of the cylinder and the boiler by the same ratio. It occurred then to James Watt that the little cylinder was exposing a greater surface to condense the steam than the cylinder of larger engines did in proportion to their respectiv contents. The model also taught him that the cylinder of the model being of brass would conduct heat much better than the cast-iron cylinders of larger engines. Robison, footnote on page 114.

[2] Doldowlod Papers. Letter Watt to Roebuck, 23 August 1765 cited in  H.W. Dickinson & R. Jenkins, James Watt and the Steam Engine.

[3] Boulton and Watt Papers (on microfilm), James Watt to William Small, 28'" January 1769, Cambridge, University Library, Part 1, Reel I, Item 4.

[4] H.W. Dickinson & R. Jenkins, James Watt and the Steam Engine, page 24.

[4] H.W. Dickinson & R. Jenkins, James Watt and the Steam Engine, page 99.

[5] Sean Bottomley, Clare Hall, The British patent system during the Industrial Revolution, 1700-1852, Dissertation at the University of Cambridge, July 2012, footnote 132 on page 127.

[6] Watt to Small on 16th to 20th March 1770 cited in H.W. Dickinson & R. Jenkins, James Watt and the Steam Engine.

[7] John Farey, page 486