DARCY (Count), an ingenious philosopher and mathematician, was born in Ireland in the year 1725; but his friends being, like many other great and good families at that period, attached to the house of Stuart, he was at 14 years of age sent to France, where he spent the rest of his life. Giving early indications of a genius for science, he was put under the care of the celebrated Clairaut (see CLAIRAUT, Encycl.), under whose tuition he improved so rapidly in the mathematics, that at 17 years
of age he gave a new solution of the problem concerning 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 a horizontal motion in the plane.
Though Darcy served in the war of 1744, he found leisure, during the bustle of a military life, to send two memoirs to the academy: the first of these contained a general principle in mechanics, that of the preservation of the rotatory motion; a principle which he again brought forward in 1750, by the name of the principle of the preservation of action. He was taken prisoner in this war by the English; and such was either the respect paid to science, or the mercy of the cabinet of St James's, that he was treated, not as an Irish rebel, but as a French subject fighting for his king and his country.
In 1760, Darcy published An Essay on Artillery, containing some curious experiments on the charges of gunpowder, &c. &c. and improvements on those of the ingenious Robins; a kind of experiments which our author carried on occasionally to the end of his life. In 1765, he gave to the public the most ingenious of all his works, his Memoir on the Duration of the Sensation of Sight; in which he endeavours to prove, and indeed completely proves, that a body may sometimes pass by our eyes without producing a sensation attended with consciousness or marking its presence, otherwise than by weakening the brightness of the object which it may chance to cover in its passage. If in this work he shall be thought to have taken hints from Dr Hartley, it is not perhaps too much to say, that some of our most celebrated writers on vision have since been beholden to Darcy. No man indeed has cause to be ashamed of being indebted to him; for all his works display in an eminent degree the union of genius and philosophy; but as he measured every thing upon the largest scale, and required extreme accuracy in experiment, neither his time, fortune, nor avocations, allowed him to execute more than a very small part of what he projected.
In his disposition, Darcy was amiable, spirited, lively, and a lover of independence; a passion to which he nobly sacrificed, even in the midst of literary society.—He died of a cholera morbus in 1779, at 54 years of age. He was admitted of the French academy in 1749, and was made pensioner-geometrician in 1770. His essays, printed in the Memoirs of the Academy of Sciences, are various and very ingenious, and are contained in the volumes for the years 1742, 1747, 1749, 1750, 1751, 1752, 1753, 1754, 1758, 1759, 1760, 1765, and in tom. 1. of the Savans Etrangers.
DATA or EUCLID, the first in order of the books that have been written by the ancient geometers, to facilitate and promote the method of resolution or analysis. In general, a thing is said to be given which is either actually exhibited, or can be found out, that is, which is either known by hypothesis, or that can be demonstrated to be known: and the propositions in the book of Euclid's data shew what things can be found out or known, from those that by hypothesis are already known: so that in the analysis or investigation of a problem, from the things that are laid down as given
or known, by the help of these propositions, it is demonstrated that other things are given, and from these last that others again are given, and so on, till it is demonstrated that that which was proposed to be found out in the problem is given; and when this is done, the problem is solved, and its composition is made and derived from the compositions of the data which were employed in the analysis. And thus the data of Euclid are of the most general and necessary use in the solution of problems of every kind.
Marinus, at the end of his preface to the data, is mistaken in asserting that Euclid has not used the synthetic, but the analytical method in delivering them: for though in the analysis of a theorem, the thing to be demonstrated is assumed in the analysis; yet in the demonstrations of the data, the thing to be demonstrated, which is, that something is given, is never once assumed in the demonstration; from which it is manifest, that every one of them is demonstrated synthetically: though indeed if a proposition of the data be turned into a problem, the demonstration of the proposition becomes the analysis of the problem. Simpson's Preface to his edition of the Data.