BOUGUER (Perms), an eminent French Mathe matician, was born in 1698. His father was King's Professor of Hydrography as Crokic in Lower lin -May, one of the best Hydrogtaphers of his time, -and author of an excellent Treatise on Navigation. rioung &tissue: was bred -to AlLethernatice *oat his infancy,. and made rapid progress in that science. k,,, At an early age he was appointed to succeed his father in the chair of Professor of Hydrography, after having undergone a strict examination in Ma thematics, so as completely to satisfy his examiners. In 1727, he gained the prize given by the Academy of Sciences of Paris, for his paper On the best Man ner of forming and distributing the Masts of Ships. He got two other prizes from the Academy in the Course of four years ; the one was bestowed on him for his Dissertation tirs the best Method of observing Elie Altitude of Stars at Sea ; the other, for. he paper On the best Method of observing the Variation of the Compass at Sea. These papers are published in the Prix de l' Academie des Sciences. In 1729, he published a work entitled Easel d'Optique stir la Gra dation de la Lumt?re ; the object of which is to define the quantity of light which is lost by passing through a given extent of the atmosphere. He finds the light of the sun to be .800 times more in tense than that of the moon.
He was soon after made Professor of Hydra grapby at Havre, whereby he had the advantage of being nearer Paris than before ; • and he was chosen associate Geometer of the Academy of Sciences, a place which did not require residence in Paris. In this place he was the successor of Maapertius. Afterwards, he was promoted in the Academy to the place of .pensioned Astronomer, and came to •ode m Paris.
It was resolved in Fiance to semi an expedition to South America for the purpose of measuring a degree of the meridian near tbe. From that measurement, compared with of a de gree vf the meridian in other latitudes, the deviation from -sphericity in the figure of the earth sight be known. The Academy made choice of titer of its members to proceed on this voyage ; they were Gedin, Bouguer, and de la Condamine, for the .geodetical epeeation, and the younger Jeans for observations in Natural History. Bouguer and hie fellow-travellers sailed from La Rochelle in 1785, and it was ten years before be returned to France. The account of his operations during the expedition its given by him in the Memoirs of the Academy 0. Sciences, T744, and in a separate work, entitled -La Figure de la Terre determinie par la observation -de MM. Bouguer et de ha Coosiese. There 'likewise an account of this expedition published by Don George Juan and Don Antonio de Moe two scientific naval officers, who accompanied the expe dition by order of the Spanish Government. The
length of a portion of the meridian was measured on the ground by means of a base and a set of triangles. Then by observing the altitude of the s of Orion which passed near the zenith, simultaneously it: the two ends of the meridian line that had been meager . ed, that fine was found to contain 3° 7' of latitude A star near the zenith wan employed, to the end that the observation might not be affected by refrectiont of Orion passed the meridian in the zenith near the middle of the line measured, so that the & tame of that star south of the zenith of the northern extremity of the line was 1° 25' 46"; and instilment worth of the zenith of the southern extremity of the line was 1° 41' 13" ; the sum of these two numbers making 3° 7'. The altitude was taken by zenith sectors of a long radius. The ground on which these operations were performed was elevated 12,000 feet above the level of the sea, and 4200 feet above the neighbouring city of Quito, and situate in a plain extending from north to south, between the two ridges of the Cordillera. The northern extre mity of the arc was on the equator. The length of the degree resulting was 56,767 toises ; but this was the degree of a curve circumscribed round the earth at the height of 12,000 feet above the level of the sea ; and the length of the degree at the level of the sea deduced from this, with some other corrections, is 56,753 toises. This length of the degree of the meridian at the equator was compared with the de gree of the meridian measured in France, with the degree measured in Lapland, and with the degree of longitude deduced in the south of France. From this comparison it was concluded, that the equato rial diameter of the earth is to the polar diameter as 179 to 178, and that the equatorial radius of the earth was about eigHt leagues longer than the polar. Since the time of Bouguer, degrees have been mea sured in different climates, with more accurate in struments than he possessed ; but the precisepro portion of' the equatorial and polar diameters of the earth is not yet finally ascertained. Bouguer makes the excess of the equatorial diameter above the polar to be 147 ; Sir Isaac Newton made it •; ; Laplace, calculating from the lunar motion, Melander hielm and Svanberg, from a degree measured anew in Lapland in 1783, compared with the degree mea sured in the province of Quito, 3ks. Bouguer found the seconds of a line shorter at the summit of thin at the level of the sea ; that is, the force of gravity was less by one 1200th part at that elevation.