er a principle which, for him, had
a generality greater than mechanics itself, and so his discovery was
in advance not only of his own time but of half the century. He may
justly be considered the founder of modern energetics.

Freed from the obscurities which prevented its being clearly
perceived, his idea stands out to-day in all its imposing simplicity.
Yet it must be acknowledged that if it was somewhat denaturalised by
those who endeavoured to adapt it to the theories of mechanics, and if
it at first lost its sublime stamp of generality, it thus became
firmly fixed and consolidated on a more stable basis.

The efforts of Helmholtz, Clausius, and Lord Kelvin to introduce the
principle of the conservation of energy into mechanics, were far from
useless. These illustrious physicists succeeded in giving a more
precise form to its numerous applications; and their attempts thus
contributed, by reaction, to give a fresh impulse to mechanics, and
allowed it to be linked to a more general order of facts. If
energetics has not been able to be included in mechanics, it seems
indeed that the attempt to include mechanics in energetics was not in
vain.

In the middle of the last century, the explanation of all natural
phenomena seemed more and more referable to the case of central
forces. Everywhere it was thought that reciprocal actions between
material points could be perceived, these points being attracted or
repelled by each other with an intensity depending only on their
distance or their mass. If, to a system thus composed, the laws of the
classical mechanics are applied, it is shown that half the sum of the
product of the masses by the square of the velocities, to which is
added the work which might be accomplished by the forces to which the
system would be subject if it returned from its actual to its initial
position, is a sum constant in quantity.

This sum, which is the mechanical energy of the system, is therefore
an invariable quantity in all the states to which it may be brought by
the interaction of its various parts, and the word energy well
expresses a capital property of this quantity. For if two systems are
connected in such a way that any change produced in the one
necessarily brings about a change in the other, there can be no
variation in the characteristic quantity of the second except so far
as the characteristic quantity of the first itself varies--on
condition, of course, that the connexions are ma