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134 were the letters exchanged between Tycho and Landgrave Wilhelm of Hesse, and his astronomer Christopher Rothmann. We have seen how Tycho's visit to Cassel in the year 1575 seems to have given a fresh impetus to the scientific tastes of the Landgrave, who in 1577 engaged Christopher Rothmann of Anhalt as his mathematicus, a man not without some knowledge of astronomy and mathematics, though not possessing the genius of the man who, two years later, was engaged as his assistant. Joost Bürgi was born in 1552, at Lichtensteig, in the county of Toggenburg, in Switzerland, and seems to have been a watchmaker in his youth, but nothing is known of his life until Landgrave Wilhelm, in 1579, appointed him court-watchmaker at Cassel. The methods of observing adopted in the observatory at Cassel rendered good clocks indispensable, and both these and the constantly improved instruments made the services of the ingenious mechanician most valuable to the Landgrave, who, indeed, was well aware what a treasure he had found, as he in one of his letters to Tycho calls Bürgi a second Archimedes. Observations were regularly made at Cassel by Rothmann and Bürgi, especially of the fixed stars, with the object of constructing a new star-catalogue, but other celestial phenomena were not altogether neglected, and the comet of 1585 gave rise to a correspondence between Tycho and the Landgrave. They had lost sight of each other since 1575, but the Landgrave was well aware that a magnificent observatory had been erected at Hveen, and that work was steadily carried on there, particularly since the visit of Wittich had put him in possession of the important improvements which Tycho had introduced in the construction of instruments. He was therefore