Mechanics as ordinarily understood is the study of reversible
phenomena. If there be given to the parameter which represents
time,[1] and which has assumed increasing values during the duration
of the phenomena, decreasing values which make it go the opposite way,
the whole system will again pass through exactly the same stages as
before, and all the phenomena will unfold themselves in reversed
order. In physics, the contrary rule appears very general, and
reversibility generally does not exist. It is an ideal and limited
case, which may be sometimes approached, but can never, strictly
speaking, be met with in its entirety. No physical phenomenon ever
recommences in an identical manner if its direction be altered. It is
true that certain mathematicians warn us that a mechanics can be
devised in which reversibility would no longer be the rule, but the
bold attempts made in this direction are not wholly satisfactory.
[Footnote 1: I.e., the time-curve.--ED.]
On the other hand, it is established that if a mechanical explanation
of a phenomenon can be given, we can find an infinity of others which
likewise account for all the peculiarities revealed by experiment.
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