As I noted in Part I, the general idea of using heat and steam for mechanical work had a long and continuous history back to ancient times. But when we talk of the breakthrough “steam engine” in the eighteenth-century sense, we don’t mean a machine that exploits steam’s expansive, or pushing force. We actually mean a machine that does the exact opposite, exploiting the apparent sucking power that occurs when hot steam is rapidly condensed with a spray of cold water. It’s the relative weight of the atmosphere, compared to the sudden vacuum from condensing steam, that does all the work.