How did the Newcomen atmospheric engine generate its working stroke?

The weight of the outside air pushing the piston down into a vacuum created beneath it.

The core principle distinguishing the Newcomen engine was its utilization of atmospheric pressure rather than relying on high internal steam pressure, which was dangerous. Steam from the boiler was first used to fill the cylinder, raising the beam. Then, a jet of cold water was injected, immediately condensing the steam and creating a near-vacuum below the piston. Since the top of the cylinder was open to the atmosphere, the massive weight of the surrounding air, approximately 14.7 pounds per square inch at sea level, exerted enormous force downwards on the piston. This downward movement was defined as the engine's working stroke, which then lifted the pump rods attached to the opposite end of the beam.