Bouguer, Pierre

Pierre Bouguer, (born Feb. 16, 1698, Le Croisic, France—died Aug. 15, 1758, Paris), versatile French scientist best remembered as one of the founders of photometry, the measurement of light intensities.

Bouguer was a prodigy trained by his father, Jean Bouguer, in hydrography and mathematics. Upon his father’s death, Pierre—at age 15—succeeded the elder Bouguer as royal professor of hydrography. During the 1720s he made some of the earliest measurements in astronomical photometry, comparing the apparent brightness of celestial objects to that of a standard candle flame. He made tables of atmospheric refraction, investigated the absorption of light in the atmosphere, and formulated Bouguer’s law (sometimes called Lambert’s law), regarding the attenuation of a light beam in a transparent medium. This law and his photometric work he published in his Essai d’optique sur la gradation de la lumière (1729; Optical Treatise on the Gradation of Light).

In 1735 he set off on an expedition with C.M. de la Condamine to measure an arc of the meridian near the Equator in Peru; he used the results obtained to make a new determination of Earth’s shape. He later gave a full account of his researches in La Figure de la terre (1749; “The Shape of the Earth”). In 1748, he built one of the first heliometers, a telescope used to measure the Sun’s diameter and the angles between celestial bodies. Bouguer measured gravity by pendulum at different altitudes and was the first to attempt to measure the horizontal gravitational pull of mountains. He observed the deviation of the force of gravity, measured on a high plateau, from that calculated on the basis of the elevation, and he correctly ascribed the effect to the mass of matter between his station and mean sea level.

Bouguer devoted much of his life to the study of nautical problems. He wrote on naval maneuvers and navigation and, in ship design, derived a formula for calculating the metacentric radius, a measure of ship stability.