How do lever machines work? I know, no pump but

How do lever machines work? I know, no pump but

But how are these units heated up? Right now I am not so much excited about the lever as I am the prospect of a kind of heat exchanger type machine. How does the boiler work? How is it heated up and what is the role of steam? Most seem to have a sight glass on

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How do lever machines work? I know, no pump but

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