, of a noble and ancient family in Ireland, was born in Galloway
, of a noble and ancient family in Ireland, was born in Galloway Sept. 18, 1725. His
parents, who were attached to the exiled house of Stuart,
sent him to Paris in 1739, where, being put under the
care of M. Clairault, at seventeen years of age he gave a
new solution of the problem of the curve of equal pressure
in a resisting medium. This was followed the year after
by a determination of the curve described by a heavy body,
sliding by its own weight along a moveable plane, at the
same time that the pressure of the body causes an horizontal motion in the plane. This problem had indeed been
solved by John Bernoulli and Clairault; but, besides that
chevalier Darcy’s method was peculiar to him, we discover
throughout the work traces of that originality which is the
leading character of all his productions. The commencement of the war took him off in some measure from his
studies, and he served during several campaigns in Germany and Flanders, as captain of the regiment of Conde.
In 1746 he was appointed to accompany the troops that
were to be sent to Scotland to assist the pretender; but the
vessel in which he sailed was taken by the English, and
Darcy, whose life was forfeited by the laws of his country,
as being taken in arms against her, was saved by the humanity of the English commander. During the course of
this war, amidst all its bustle and dangers, he found leisure to contribute two memoirs to the academy. The first
contained a general principle of mechanics, that of the
preservation of the rotatory motion. Daniel Bernoulli and
Euler had found it out in 1745; but, besides that it is not
likely their works should have reached Mr. Darcy in the
midst of his campaigns, his method, which is different from
theirs, is equally original, simple, elegant, and ingenious.
This principle, which he again brought forward in 1750,
by the name of “the principle of the preservation of action,
” in order to oppose it to Maupertuis’s principle of the
least action, Darcy made use of in solving the problem of
the precession of the equinoxes: here, however, he miscarried; and in general it is to be observed, that though
all principles of this kind may be used as mathematical formuloe, two of them at least must necessarily be employed
in the investigation of problems, and even these with great
caution; so that the luminous and simple principle given
by M. d'Alembert in 1742 is the only one, on account of
its being direct, which can be sufficient of itself for the sotion of problems.