DARCY (Count)

, an ingenious, philosopher and mathematician, was born in Ireland in 1725; but his friends being attached to the Stuart family, he was sent to France, at 14 years of age, where he spent the rest of his life. Being put under the care of the celebrated Clairaut, 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. Darcy served in the war of 1744, and was taken prisoner by the English: and yet, during the course of the war he gave 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.

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 published his Memoir on the Duration of the Sensation of Sight, the most ingenious of his works: the result of these researches was, that a body may sometimes pass by our eyes without being seen, or marking its presence, otherwise than by weakening the brightness of the object it covers.

All Darcy's works bear the character which results from 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, he 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.

Darcy 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.

DARK Chamber. See Camera Obscura.

DARK Tent, a portable camera obscura, made somewhat like a desk, and fitted with optic glasses, to take prospects of landscapes, buildings, &c.