330IIGI7ER, PIERRE, one of the most eminent French mathematicians and natural phi losophers of his time, was born at Croisie, in Bretagne, 16th Feb., 1698. and studied in the Jesuit college at Vannes. In 1713, he succeeded his father as professor of hydrog raphy in Croisic, from which he was removed to a similar office at Hayre in 1730. In 1720, he published his Essai irOptirrle la Gradation de la Lumare. Ills researches on other subjects of natural philosophy and astronomy continued to add to his fame: and in 1731, be was made associate geometer of the Academy of Sciences, and promoted to the office of pensioned astronomer in 1735. In that year, also, he was chosen to pro ,cced, alonr• with Godin and De la Condamine, to South America. in order to the meas urement of a degree of the meridian at the equator. 13. and his companions had to contend with many difficulties, and were more than seven years away from home, during which time 13. made _valuable observations on the length of the seconds' pendulum at great elevations, the.deviatioa of the pltimb-line .from n vertical position through the
attraction of a neighboring mountain, the limit of perpetual snow, the obliquity of the ecliptic, etc. Ire published an account of his labors and those of Ilis colleagues in a magnificent work. entitled La Figure de la Terre daerminee par MAT. Ilmiguer et De la Condamine (Par. 1749), which, however, involved him in an unpleasant controversy with de la Condamine concerning their respective slinres of merit in the researches in which they had been jointly engaged. 13.'s investigations concerning the intensity of light laid 'the foundation of photometry; and their results, which had been partly exhibitea in the Optical work already noticed, were more fully embodied in his Teallei d'Optique sur la Gradation de la Lezni?re, which was edited after his death by Laud& (Par. 1760). Ile invented the heliometer in 1748, which has of late been brought to greater perfection by Frannhofer. He also published an excellent work on navigation (Par. 1753). Ile died in 1758.