Lazare Carnot

Although he always worked on the fringes of the scientific world of his time,
Sadi Carnot did not otherwise live in obscurity. His father, Lazare, was one of
the most powerful men in France during the late eighteenth and early nineteenth
centuries. Sadi was born in 1796 in the Paris Luxembourg Palace when Lazare
was a member of the five-man executive Directory. Lazare Carnot served in highlevel
positions for only about four years, but his political accomplishments and
longevity were extraordinary for those turbulent times. Before joining the government of the Directory, he was an influential member of the all-powerful Committee
of Public Safety led by Maximilien de Robespierre. In that capacity, Lazare
was responsible for the revolutionary war efforts. His brilliant handling of logistics
and strategy salvaged what might otherwise have been a military disaster; in
French history textbooks he is known as “the great Carnot” and “the organizer
of victory.” He was the only member of the Committee of Public Safety to survive
the fall of Robespierre in 1794 and to join the Directory. A leftist coup in 1797
forced him into exile, but he returned as Napoleon’s war minister. (He had given
Napoleon command of the Italian army in 1797.) Napoleon’s dictatorial ways
soon became evident, however, and Lazare, unshakable in his republican beliefs,
resigned after a few months. But he returned once more in 1814, near the end of
the Napoleonic regime, first as the governor of Antwerp and then as Napoleon’s
last minister of the interior.
Lazare Carnot’s status in history may be unique. Not only was he renowned
for his practice of politics and warfare; he also made important discoveries in
science and engineering. A memoir published in 1783 was, according to Lazare’s
biographer, Charles Gillispie, the first attempt to deal in a theoretical way with
the subject of engineering mechanics. Lazare’s goal in this and in later workin
engineering science was to abstract general operating principles from the mechanical
workings of complicated machinery. His aim, writes Gillispie, “was to
specify in a completely general way the optimal conditions for the operation of
machines of every sort.” Instead of probing the many detailed elements of machinery
design, as was customary at the time, he searched for theoretical methods
whose principles had no need for the details.
Lazare Carnot’s main conclusion, which Gillispie calls the “principle of continuity
of power,” asserts that accelerations and shocks in the moving parts of
machinery are to be avoided because they lead to losses of the “moment of activity”
or workoutput. The ideal machine is one in which power is transmitted
continuously, in very small steps. Applied to water machines (for instance, waterwheels),
Lazare’s theorem prescribes that for maximum efficiency there must
be no turbulent or percussive impact between the water and the machine, and
the water leaving the machine should not have appreciable velocity.
Lazare’s several memoirs are not recognized today as major contributions to
engineering science, but in an important sense his worksurvives. His approach
gave his son Sadi a clear indication of where to begin his own attackon the
theory of heat engines. Lazare’s views on the design of water engines seem to
have been particularly influential. Waterwheels and other kinds of hydraulic machinery
are driven by falling water, and the greater the fall, the greater the machine’s
workoutput per unit of water input. Sadi Carnot’s thinking was guided
by an analogy between falling water in water engines and falling heat in heat
engines: he reasoned that a heat engine could not operate unless its design included
a high-temperature body and a low-temperature body between which heat
dropped while it drove the working parts of the machine.