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. It should be noted, in
addition, that it is immaterial what liquid there may be in the lake;
whether water, oil, mercury, or what not, the law will equally apply,
and so in a heat engine, the nature of the working substance, provided
that it does not change its physical state during a cycle, does not
affect the question of efficiency with which the heat being expended
is so utilized. To make this matter clearer, and give it a practical
bearing, I will give the symbols a numerical value, and for this
purpose I will, for the sake of simplicity, suppose that the fuel used
is pure carbon, such as coke or charcoal, the heat of combustion of
which is 14,544 units, that the specific heat of air, and of the