The mean sea level is the lowest level
to which the water can possibly fall; hence its greatest potential
energy, that of its position in the lake, = QT = H. The water is
working between the absolute levels, T and _t_; hence, according to
Carnot, the maximum effect, W, to be expected is--
/ T - t \
W = H( ------- )
\ T /
/ T - t \
but H = QT [therefore] W = Q T( ------- )
\ T /
W = Q (T - t),
that is to say, the greatest amount of work which can be expected is
found by multiplying the weight of water into the clear fall, which
is, of course, self-evident.
Now, how can the quantity of work to be got out of a given weight of
water be increased without in any way improving the efficiency of the
turbine? In two ways:
1. By collecting the water higher up the mountain, and by that means
increasing T.
2. By placing the turbine lower down, nearer the sea, and by that
means reducing _t_.
Now, the sea level corresponds to the absolute zero of temperature,
and the heights T and _t_ to the maximum and minimum temperatures
between which the substance is working; therefore similarly, the way
to increase the efficiency of a heat engine, such as a boiler, is to
raise the temperature of the furnace to the utmost, and reduce the
heat of the smoke to the lowest possible point.
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