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" A certain
gentleman owns a little pond, fed by one stream and drained by
another. The pond also receives water from an occasional rainfall and
loses it by evaporation, which we will consider as "negative rain".
In the analogy we wish to pursue the pond is our system, the water
within it is the internal energy, water transferred by the streams is
work, and water transferred as rain is heat." Callen develops the
analogy for the best part of a page -the first point is that no
examination of the pond at any time can indicate how much of the water
came from the streams and how much by rain.