In the ordinary steam engine both the recovery of work and changes of the working substance involve the motion of a piston. Two heat engines are described here in which the gravitational field permits a separation of these two aspects. The first machine evaporates a liquid (for example, water) in the heat source, condenses it in the sink, and work is obtained from a water wheel; this machine is easy to understand, though not very efficient. The second machine is a reversible one using a Carnot cycle, again separating volume changes of the working substance from the motion of a piston. Both machines require large vertical dimensions but may someday be feasible in the exploitation of geothermal energy.

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