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206 Rothmann, who came to Hveen on the 1st August and stayed with Tycho till the 1st September. He seems to have been a somewhat peculiar character, and to have taken a rather unfair advantage of the great modesty and retiring disposition of his colleague, Bürgi, to push himself into the foreground, if not actually to try to shine with borrowed plumes. In a letter of which he was the bearer, the Landgrave wrote that Rothmann had been in bad health for some time, and imagined that a little travelling and change of air would do him good. "But he has a head of his own, for which he every year buys a hat of his own, so we must leave him to himself; but we should be sorry if anything happened to him, for he is ingenious and a fine, learned fellow." Tycho and he had now been in regular correspondence for about four years, and had in their letters entered very fully into the methods of observing used at Hveen and at Cassel, and the advantages or difficulties of the Copernican system. They had discussed the frequently observed "chasmata" or auroræ (which Tycho took to be sulphurous exhalations ignited in the air, and not clouds illuminated by the sun, as the latter was too far below the horizon in winter ); they had exchanged ideas about the celestial space, which Tycho did not believe to be filled with air, as this would produce a sound when the planets moved through it (which Rothmann denied); about the amount of refraction and the duration of twilight, for which Rothmann assumed a depression of the sun equal to 24°, while Tycho found 16° to 17°, and on any other subject which their work might suggest. Rothmann had prepared for