n the relation between the work
done and the heat spent in doing it. His other idea of the reversible
cycle implies that an engine is reversible when, instead of using heat
and getting work from it, the engine may be driven through the cycle of
operations the reverse way, that is, by taking in work, it can pump back
heat to the boiler again. Carnot showed that if you can obtain such a
reversible engine, it is a perfect engine. All perfect engines, that is
all reversible engines, will do exactly the same amount of work with the
same amount of heat, the amount of work being strictly proportionate to
the amount of heat consumed. I need hardly point out that the reversible
engine, or the perfect engine of Carnot, is only the ideal one, as there
is no engine in which all the heat is converted into work, as a great
deal of the heat is radiated away and not converted into work at all.
Again, working from the standpoint that heat is matter, Carnot reasoned
that in the heat-engine the work is performed, not by the actual
consumption of heat, but by its transportation from a hot body to a cold
one. Thus, by the fall of heat from a higher to a lower temperature,
work could be done in the same way that work could be done by allowing
water to fall from a higher to a lower level. The quantity of water
which reaches the lower level is exactly the same as that which leaves
the higher level, as none of the water is destroyed in the fall. He
argued, therefore, that the work produced by a heat-engine was produced
in a similar manner, the quantity of heat which reaches the condenser
being supposed to be equal to that which left the source. Thus the work
was done by the heat flowing from a hot body to a cold one, and, in
doing this work, it lost its momentum like falling water, and was
brought to rest. One of the most important points noted by Carnot is
the necessity that, in all engines which derive work from heat, there
must be two bodies at different temperatures, that is, a source and a
condenser, which correspond to a hot and cold body, so that there may be
the passage of heat from the hot to the cold body. In order to get work
out of heat it is absolutely necessary to have a hotter and a colder
body. From this reasoning we learn, therefore, that work is obtained
from heat by using up the heat of the hotter body, part of which is
converted into actual work, while part is absorbed by the colder body.
So that wherever we have two bodies at