The Newcomen Beam or Atmospheric Engine at Elsecar New Colliery is the only surviving atmospheric engine of its type still in its original operational site. In 1794 the building materials were gathered and construction of the engine house began. By September 1795 the engine was complete and began working. In 1801 it was fitted with a new cylinder and the beam was replaced with the present cast iron version in 1836. It was in service until 1923 when it was replaced by electric pumps. It was used as a standby until 1930 and then only for demonstrations until the early 1950s. The Newcomen Beam Engine was bought by Barnsley Metropolitan Borough Council in 1988 and a program of conservation and restoration was begun.
The engine is housed in a three storey stone engine house with slate roof. The large iron beam is accommodated, half in and half out, on the west side of the top storey. The second and third floors have a central well to house the cylinder and piston and each floor is accessed via a wooden staircase. A platform runs from the engine house to the adjacent gantry alongside the beam on the top floor. The engine consists of a vertical cylinder with a piston connected by a chain to one end of a large rocking beam. Chains descended from the other end of the beam into the mine. The original rocking beam was made of timber, however, it was replaced by a cast-iron one in 1836. It is 24ft long and 4ft wide at its centre and the cylinder measures 4ft in diameter with a stroke of 5ft. The boilers were originally "haystack" type, later replaced by the Cornish externally fired type. Steam was then used to power the engine, supplied by a modern boiler from nearby workshops. The engine worked by injecting water directly into the cylinder to condense the steam within and therefore drive, under atmospheric pressure, the piston above. At maximum output the engine could achieve 6-8 strokes per minute and lift 50 gallons of water from the mine with each stroke.