Jean-André Deluc (8 February 1727 - 7 November 1817) was a Swiss geologist and meteorologist.

He was born at Geneva, descended from a family which had emigrated from Lucca and settled at Geneva in the 15th century. His father, François Deluc, was the author of some publications in refutation of Mandeville and other rationalistic writers, which are best known through Rousseaus humorous account of his ennui in reading them; and he gave his son an excellent education, chiefly in mathematics and natural science. On completing it Jean-André engaged in commerce, which principally occupied the first forty-six years of his life, without any other interruption than that which was occasioned by some journeys of business into the neighboring countries, and a few scientific excursions among the Alps.

During these, however, he collected by degrees, in conjunction with his brother Guillaume Antoine Deluc, a splendid museum of mineralogy and of natural history in general, which was afterwards increased by his nephew J. André Deluc (1763-1847), who was also a writer on geology. He at the same time took a prominent part in politics. In 1768 he was sent to Paris on an embassy to the duc de Choiseul, whose friendship he succeeded in gaining. In 1770 he was nominated one of the Council of Two Hundred.

Three years later unexpected reverses in business made it advisable for him to quit his native town, which he only revisited once for a few days. The change was welcome in so far as it set him entirely free for scientific pursuits, and it was with little regret that he removed to England in 1773. He was made a fellow of the Royal Society in the same year, and received the appointment of reader to Queen Charlotte, which he continued to hold for forty-four years, and which afforded him both leisure and a competent income.

In the latter part of his life he obtained leave to make several tours in Switzerland, France, Holland and Germany. In Germany he passed the six years from 1798 to 1804; and after his return he undertook a geological tour through England. When he was at Gottingen, in the beginning of his German tour, he received the compliment of being appointed honorary professor of philosophy and geology in that university; but he never entered upon the active duties of a professorship. He was also a correspondent of the Academy of Sciences at Paris, and a member of several other scientific associations. He died at Windsor in 1817.

His favorite studies were geology and meteorology. The situation of his native country had naturally led him to contemplate the peculiarities of the earths structure, and the properties of the atmosphere, as particularly displayed in mountainous countries, and as subservient to the measurement of heights. According to Georges Cuvier, he ranked among the first geologists of his age. His principal geological work, Lettres physiques et morales sur les montagnes et sur l/iistoire de la terre et de lhornme, first published in 1778, and in a more complete form in 1779, was dedicated to Queen Charlotte. It dealt with the appearance of mountains and the antiquity of the-human race, explained the six days of the Mosaic creation as so many epochs preceding the actual state of the globe, and attributed the deluge to the filling up of cavities supposed to have been left void in the interior of the earth. He published later an important series of volumes on geological travels in the north of Europe (1810), in England (1811), and in France, Switzerland and Germany (1813). These were translated into English.

Deluc's original experiments relating to meteorology were valuable to the natural philosopher; and he discovered many facts of considerable importance relating to heat and moisture. He noticed the disappearance of heat in the thawing of ice about the same time that J. Black founded on it his ingenious hypothesis of latent heat. He ascertained that water was more dense about 40 F. (4 C.) than at the temperature of freezing, expanding equally on each side of the maximum; and he was the originator of the theory, afterward readvanced by John Dalton, that the quantity of aqueous vapour contained in any space is independent of the presence or density of the air, or of any other elastic fluid.

His Recherches sur les modifications de latmosphre (2 vols. 4to, Geneva, 1772; 2nd ed., 4 vols. 8vo, Paris, 1784) contains many accurate and ingenious experiments upon moisture, evaporation and the indications of hygrometers and thermometers, applied to the barometer employed in determining heights. In. the Phil. Trans., 1773, appeared his account of a new hygrometer, which resembled a mercurial thermometer, with an ivory bulb, which expanded by moisture, and caused the mercury to descend. The first correct rules ever published for measuring heights by the barometer were those he gave in the Phil. Trans., 1771, p. 158. His Lettres sur lhistoire physique de la terre (8vo, Paris, 1798), addressed to Professor Johann Friedrich Blumenbach, contains an essay on the existence of a General Principle of Morality. It also gives an interesting account of some conversations of the author with Voltaire and Rousseau. Deluc was an ardent admirer of Bacon, on whose writings he published two works; Bacon tel quil est (8vo, Berlin, 1800), showing the bad faith of the French translator, who had omitted many passages favorable to revealed religion, and Prcis de la philosophic de Bacon (2 vols 8vo, Paris, 1802), giving an interesting view of the progress of natural science. Lettres sur le Christianisme (Berlin and Hanover, iSo,, 1803) was a controversial correspondence with Dr Teller of Berlin in regard to the Mosaic cosmogony. His Trail lmentaire de geologie (8vo, Paris, 1809, also in English, by de la Fite, the same year) was principally intended as a refutation of the Vulcanian system of Hutton and Playfair, who deduced the changes of the earth's structure from the operation of fire, and attributed a higher antiquity to the present state of the continents than is required in the Neptunian system adopted by Deluc after Dolomieu. He sent to the Royal Society, in 1809, a long paper on separating the chemical from the electrical effect of the pile, with a description of the electric column and aerial electroscope, in which he advanced opinions so little in unison with the latest discoveries of the day, that the council deemed it inexpedient to admit them into the Transactions. The paper was afterwards published in Nicholsons Journal (xxvi.), and the dry column described in it was constructed by various experimental philosophers. This dry pile or electric column has been regarded as his chief discovery.

Many other of his papers on subjects kindred to those already mentioned are to be found in the Transactions and in the Philosophical Magazine. See Philosophical Magazine (November 1817).