Newcomen raised low-pressure steam in a domed boiler, and fed it via a pipe into the base of a metal cylinder. This broke a vacuum and freed a piston to rise inside the cylinder, lifted by the considerable weight of a pumping mechanism attached to the opposite end of a balanced beam.

At the top of the piston's upward stroke, cold water was sprayed into the cylinder. This condensed the steam into a few water droplets, leaving a vacuum where the steam had been.

Now, simple atmospheric pressure acting on the top of the open cylinder forced the piston down again, completing the cycle.

It was this downward stroke, rather than the upward one, that was the power stroke, so the device became known as an atmospheric engine.

Although the development of the engine is reasonably well documented, there are some unanswered questions about its gestation, including particularly the role of Sir Robert Hooke who, as curator of experiments at the Royal Society was the world's first truly professional scientist.

Newcomen is known to have consulted Hooke, who had a good knowledge of earlier work on the problem - particularly that by his assistant, the previously-mentioned Dionysus Papin. Hooke may well have pointed the Devon man in the right direction, after first attempting to dissuade him from the project.

Newcomen's engine had more than a few drawbacks, notably its inefficiency. It was expensive to run because the cold-water spray, besides condensing the steam, also cooled the cylinder which had to be heated all over again at the next cycle. Constant heating and cooling meant the boiler required more fuel, but this was not a problem where the engine was designed to be used - in coalmines.

The first Newcomen engine went into operation at a mine at Dudley Castle in Staffordshire in 1712. Its 21-inch diameter cylinder had a length of almost eight feet and at twelve strokes