Pierre Bouguer – Child Prodigy and ‘Father of Photometry’

Pierre Bouguer

Pierre Bouguer (1698-1758)

On February 16, 1698, French mathematician, geophysicist, geodesist, and astronomerPierre Bouguer was born. In 1735 Bouguer sailed with Charles Marie de La Condamine on a scientific mission to Peru, in order to measure a degree of the meridian arc near the equator. He is also known as “the father of naval architecture” and the “father of photometry“.

Pierre Bouguer was educated in mathematics and hydrography by his father Jean Bouguer, who was a Royal Professor of Hydrography. It is believed that Bouguer was a real child prodigy who had a deep understanding of mathematics at the age of only 15. In the same year Jean passed away and a new professor of hydrography was sought. Pierre applied for the position and due to his brilliance and great knowledge, he was appointed.

Pierre Bouguer had a great career ahead, winning the Prix of the Académie Royale des Sciences for the first time in 1727 for his work on masts of ships. The second was awarded to Bouguer for his work on the altitudes or stars at the sea and he earned the third for his research on the magnetic declination at sea.

In 1735, Pierre Bouguer joined an expedition to Peru in order to measure the length of a degree of meridian at the equator. The expedition was organised by the Académie Royale des Sciences. Louis Godin was the leader of the expedition and they head to Quito while La Condamine, who was also a member of this expedition went overland from Malta. The men met at Quito and while Godin began to work on his own, Bouguer and La Condamine worked jointly.

It is believed that Bouguer was one of the first to attempt to measure the Earth’s density with the help of the ‘deflection of a plumb line due to the attraction of a mountain’. Pierre Bouguer published his research results in La Figure de la terre.

Another field of research, Bouguer was active in, was photometry. He developed the theory that even though the eye could not detect the ‘amount’ of brightness very well, it could indeed detect if two objects had the same brightness. Therefore, he proceeded to compare the brightness of the moon to a candlelight. That method along with Kepler’s inverse square law was then used to measure brightness. The procedure was published in Essai d’optique sur la gradation de la lumière. Through this work, Bouguer’s law became famous, expressing ‘the relationship between the absorption of radiant energy and the absorbing medium’.

In 1746 he published the first treatise of naval architecture, Traité du navire, which among other achievements first explained the use of the metacenter as a measure of ships’ stability. His later writings were nearly all upon the theory of navigation and naval architecture. In January 1750 he was elected a Fellow of the Royal Society.

In the field of geodesy he wrote the work Traité de navigation (Paris 1753), which was substantially supplemented by Lacaille in the second (1769) and by Lalande in the third edition (1792). He made his first observations at all in the vicinity of the Chimborazo river by the deviation of the lead solder in the earth’s gravity field due to the attraction of the mountains and the height of the snow line.

 

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