Newcomen’s Engine: The Pumping Stroke

Put the drawing and the object on separate dates

The Science Museum’s catalog describes Henry Beighton’s 1717 engraving of the Griff engine erected in 1714. Its dates refer to a print and a pictured installation, respectively. A different record at National Museums Scotland describes the engine ordered for Caprington in 1811. Both help explain the principle. They do not depict a single unchanged object.

Look at the beam as a connection between a cylinder and a pump. Its use makes sense only if you trace both sides of the pivot. Mark the piston’s working direction first and then ask what happens at the pump end.

Sources for this section: [1] · [2] · [3]

Follow a pressure change through the beam

Steam enters the cylinder. Condensation reduces the pressure below the piston. With higher pressure outside, the piston moves down during the useful stroke. The beam converts that movement into a lift of the pump rods at its other end. The sequence separates a change in the fluid inside the cylinder from the motion of solid parts.

Try drawing the mechanism with the beam held fixed. The pressure difference could still load the piston, but without movement the pump would receive no useful stroke. The model helps show why a vacuum alone does not drain a mine.

Sources for this section: [1] · [2]

Use an invented force example

If the outside pressure is 10 units and the inside pressure is 3, the difference is 7. For a teaching piston with 2 units of area, this gives 14 force units before other loads. That is a calculation for the piston, not the pump. To estimate the pump force, you would also need the relative arm lengths and the loads in the beam and rods.