A body of water, no matter how vast, will not produce motive power unless it can flow downhill. So, too, even a prodigious quantity of heat will not create motive power if there’s no temperature difference it can “flow down.” A steam engine inside a vast hot furnace will not work despite the presence of abundant heat because there is no way to cool and liquefy the steam so the piston can be pushed back to the top of the cylinder. Carnot writes: “The production of heat alone is not sufficient to give birth to the impelling power: it is necessary that there should also be cold; without it, the
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