JESSE RAMSDEN (1735-1800), English astronomical instrument maker, was born at Salterhebble near Halifax, Yorkshire, on the 6th of October 1735. After serving his apprenticeship with a cloth-worker in Halifax, he went in 1 755 to London, where in 1758 he was apprenticed to a mathematical instrument maker. About four years afterwards he started business on his own account and secured a great reputation with his products. He died at Brighton on the 5th of November 1800. Ramsden's speciality was divided circles, which began to supersede the quadrants in observatories towards the end of the 18th century. His most celebrated work was a 5-feet vertical circle, which was finished in 1789 and was used by G. Piazzi at Palermo in constructing his well-known catalogue of stars. He was the first to carry out in practice a method of reading off angles (first suggested in 1768 by the duke of Chaulnes) by measuring the distance of the index from the nearest division line by means of a micrometer screw which moves one or two fine threads placed in the focus of a microscope. Ramsden's transit instruments were the first which were illuminated through the hollow axis; the idea was suggested to him by Prof. Henry Ussher in Dublin. He published a Description of an Engine for dividing Mathematical Instruments in 1777.
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