He could be said to have been more than a lay preacher as he was a teaching elder in the local Baptist church. That he continued in business is almost certainly because the church could not afford to pay him as a full time elder. His father had been one of a group who brought the well known Puritan John Flavel to Dartmouth. Later one of Newcomen's business contacts in London, Edward Wallin, was another baptist minister who had connections with the well known Dr John Gill of Horsleydown, Southwark. Newcomen's connection with the Baptist church at Bromsgrove materially aided the spread of his steam engine.

Newcomens dampmaskine Newcomen's great achievement was his steam engine, probably developed about 1710, combining the ideas of Thomas Savery and Denis Papin. It is likely that Newcomen was already acquainted with Savery, whose forebears were merchants in south Devon in 1712. Savery also had a post with the Commissioners for Sick and Hurt Seamen, which took him to Dartmouth. Savery had devised a 'fire engine', a kind of thermic syphon, in which steam was admitted to an empty container and then condensed. The vacuum thus created was used to suck water from the sump at the bottom of the mine. The 'fire engine' was not very effective and could not work beyond a limited depth of around thirty feet.

Newcomen replaced the receiving vessel (where the steam was condensed) with a cylinder containing a piston. Instead of the vacuum drawing in water, it drew down the piston. This was used to work a beam engine, in which a large wooden beam rocked upon a central fulcrum. On the other side of the beam was a chain attached to a pump at the base of the mine. As the steam cylinder was refilled with steam, readying it for the next power stroke, water was drawn into the pump cylinder and expelled into a pipe to the surface by the weight of the machinery. Newcomen and his partner John Calley built one of the first engines at the Conygree Coalworks near Dudley in the West Midlands. A working replica of this engine can be seen at the Black Country Living Museum nearby.

Comparatively little is known of Newcomen's later life. In his later life (at least), the engine affairs were conducted through an unincorporated company, the 'Proprietors of the Invention for Raising Water by Fire'. Its secretary and treasurer was John Meres, clerk to the Society of Apothecaries in London. That society formed a company which had a monopoly on supplying medicines to the Navy providing a close link with Savery, whose will he witnessed. The Committee of the Proprietors also included Edward Wallin, a Baptist of Swedish descent; and pastor of a church at Maze Pond, Southwark. Newcomen died at his house in 1729, and his body was buried at Bunbury Fields.

By the time of his death, about 75 of his engines, operating under Savery's patent (extended by statute so that it did not expire until 1733), had been installed by Newcomen and others in most of the important mining districts of Britain: draining coal mines in the Black Country, Warwickshire and near Newcastle upon Tyne; at tin and copper mines in Cornwall; and in lead mines in Flintshire and Derbyshire, amongst other places.

The Newcomen engine held its place without material change for about three-quarters of a century, spreading gradually to more and more areas of the UK and to mainland Europe. At first brass cylinders had been used but these were expensive and limited in size. New iron casting techniques pioneered by the Coalbrookdale Company in the 1720s allowed bigger and bigger cylinders to be used, up to about 6 feet (1.8 metres) in diameter by the 1760s, and experience gradually led to better construction and minor refinements in layout. Its mechanical details were much improved by John Smeaton, who built many large engines of this type in the early 1770s; his improvements were rapidly adopted. By 1775 about 600 Newcomen engines had been built, although many of these had worn out before then, and been abandoned or replaced.

The Newcomen Engine was by no means an efficient machine, although it was probably as complicated as engineering and materials techniques of the early eighteenth century could support. Much heat was lost when condensing the steam, as this cooled the cylinder. This did not matter unduly at a colliery, where unsaleable small coal (slack) was available, but significantly increased the mining costs where coal was not readily available, as in Cornwall. Therefore, Newcomen's engine was gradually replaced after 1775 in areas where coal was expensive (especially in Cornwall) by an improved design, invented by James Watt, in which the steam was condensed in a separate condenser. The Watt steam engine, aided by better engineering techniques including Wilkinson's boring machine, was much more fuel efficient, enabling Watt and his partner Matthew Boulton to collect substantial royalties based on the fuel saved.

Watt subsequently made other improvements, including the double-acting engine, where both the up and down strokes were power strokes. These were especially suitable for driving textile mills, and many Watt engines were employed in these industries. At first attempts to drive machinery by Newcomen engines had mixed success, as the single power stroke produced a jerky motion, but use of flywheels and better engineeering largely overcame these problems. By 1800 hundreds of non-Watt rotary engines had been built, especially in collieries and ironworks where irregular motion was not a problem but also in textile mills. (see reference (2) below).

Despite Watt's improvements, Common Engines (as they were then known) remained in use for a considerable time, and many more Newcomen engines than Watt ones were built even during the period of Watt's patent (up to 1800), as they were cheaper and less complicated: of over 2,200 engines built in the eighteenth century, only about 450 were Watt engines. Elements of Watt's design, especially the Separate Condenser, were incorporated in many „pirate“ engines. Even after 1800 Newcomen type engines continued to be built and condensers were added routinely to these. They were also commonly retro-fitted to existing Newcomen engines (the so-called „pickle-pot“ condenser).

There are examples of Newcomen engines in the Science Museum (London) and the Ford Museum, Dearborn amongst other places. Perhaps the last Newcomen-style engine to be used commercially—and the last still remaining on its original site—is at Elsecar, near Barnsley in South Yorkshire.

• L. T. C. Rolt and J. S. Allen, The Steam Engine of Thomas Newcomen (Landmark Publishing, Ashbourne 1997).
• John Kanefsky and John Robey, “Steam Engines in 18th-Century Britain: A Quantitative Assessment”, Technology and Culture, 21 (1980), 161-186
• A.E. Musson and Eric Robinson, Science and Technology in the Industrial Revolution (Manchester University Press, 1969), esp. chapter XII