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1742 __ Heliostat
Willem Jakob van's Gravesande (1688-1742)
Comment : The heliostat, a device by which a flash may be sent in any desired direction, regardless of the sun's motion, was invented by Gravesande, a Dutch physicist. The discovery of the art of making mirrors by silvering one side of a glass made greater distances possible with the heliograph. Early in the nineteenth century Johann K.F. Gauss, the German mathematician, discovered by experiments that the flash from a mirror one inch square could be seen 7 miles. With a much larger mirror the distance could be enormously increased. But little was done with the heliograph until the Morse dot-and-dash code was devised. In 1861, the United States Coast Survey, testing a mirror equatorially mounted in the Lake Superior region, found that they could send signals as much as 90 miles. By 1890 the army, working from mountain peaks in Arizona, had been able to flash a message 215 miles. The British took it up, and added night signaling with an electric or calcium light, the dots and dashes being made by alternately exposing and masking the light. They employed the heliograph in the Afghan and Boer Wars, and it was also extensively used in the World War. (Alvin H. Harlow)
French comment : Un héliostat (de helios, mot grec signifiant Soleil, et stat, racine de stationnaire) est un dispositif pour suivre la course du Soleil, typiquement pour orienter toute la journée les rayons solaires vers un point ou à une petite surface fixe, à l'aide de miroirs. Créé au XVIIIe siècle, bien qu'il n'en soit pas l'inventeur, la plus ancienne mention de l'héliostat se trouve dans un livre datant de 1742 du physicien néerlandais Willem Jacob's Gravesande. La rotation du miroir de l'héliostat est assuré par un mécanisme d'horlogerie. L'héliostat a été perfectionné par plusieurs personnes, dont le physicien français Jean Thiébault Silbermann. (Compiled from various sources)
Source : Harlow, Alvin F. (1936), "Old Wires and New Waves- The History of the Telegraph, Telephone and Wireless", READ BOOKS, 2008, pp. 10-11.
Urls : http://www.youtube.com/watch?v=HP3ZWTkkNTU (last visited )

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