What was the primary mechanism driving the piston down in the Newcomen Atmospheric Engine?

Answer

Atmospheric pressure acting on the piston top after condensation

In the Newcomen design, steam created pressure to raise the piston, but when condensed, a vacuum formed, allowing the external atmospheric pressure acting on the top of the piston to push it down, powering the pump rod.

What was the primary mechanism driving the piston down in the Newcomen Atmospheric Engine?
inventionHistoryfathersteam engine