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The Schiehallion experiment was an 18th-century experiment to determine the mean density of the Earth. Funded by a grant from the Royal Society, it was conducted in the summer of 1774 around the Scottish mountain of Schiehallion, Perthshire. The experiment involved measuring the tiny deflection of a pendulum due to the gravitational attraction of a nearby mountain. Schiehallion was considered the ideal location after a search for candidate mountains, thanks to its isolation and almost symmetrical shape. One of the triggers for the experiment were anomalies noted during the survey of the Mason–Dixon Line. The experiment had previously been considered, but rejected, by Isaac Newton as a practical demonstration of his theory of gravitation. However, a team of scientists, notably Nevil Maskelyne, the Astronomer Royal, were convinced that the effect would be detectable and undertook to conduct the experiment. The deflection angle depended on the relative densities and volumes of the Earth and the mountain: if the density and volume of Schiehallion could be ascertained, then so could the density of the Earth. Once this was known, then this would in turn yield approximate values for those of the other planets, their moons, and the Sun, previously known only in terms of their relative ratios. As an additional benefit, the concept of contour lines, devised to simplify the process of surveying the mountain, later became a standard technique in cartography. ==Background== A pendulum hangs straight downwards in a symmetrical gravitational field. However, if a sufficiently large mass such as a mountain is nearby, its gravitational attraction should pull the pendulum's plumb-bob slightly out of true. The change in plumb-line angle against a known object—such as a star—could be carefully measured on opposite sides of the mountain. If the mass of the mountain could be independently established from a determination of its volume and an estimate of the mean density of its rocks, then these values could be extrapolated to provide the mean density of the Earth, and by extension, its mass. Isaac Newton had considered the effect in the ''Principia'', but pessimistically thought that any real mountain would produce too small a deflection to measure.〔 Translated: Andrew Motte, First American Edition. New York, 1846〕 Gravitational effects, he wrote, were only discernible on the planetary scale.〔 Newton's pessimism was unfounded: although his calculations had suggested a deviation of less than 2 minutes of arc (for an idealised three-mile high mountain), this angle, though very slight, was within the theoretical capability of instruments of his day. An experiment to test Newton's idea would both provide supporting evidence for his law of universal gravitation, and estimates of the mass and density of the Earth. Since the masses of astronomical objects were known only in terms of relative ratios, the mass of the Earth would provide reasonable values to the other planets, their moons, and the Sun. The data were also capable of determining the value of Newton's gravitational constant , though this was not a goal of the experimenters; references to a value for would not appear in the scientific literature until almost a hundred years later. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Schiehallion experiment」の詳細全文を読む スポンサード リンク
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