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Rh In each of the two small observatories there was an equatorial armillary sphere, of which the northern one was ornamented with pictures of Copernicus and Tycho himself. In the large southern observatory were the following instruments. A vertical semicircle (eight feet in diameter) turning round a vertical axis, and furnished with a horizontal circle for measuring azimuths (fol. B. 5); a triquetrum, or, as Tycho calls it, "instrumentum parallacticum sive regularum" (fol. C.); a sextant for measuring altitudes with a radius of 5$1⁄2$ feet (fol. A. 5); and a quadrant of two feet radius with an azimuth circle (fol. A. 4). In the large northern observatory were another triquetrum of peculiar construction, with an azimuth circle 16 feet in diameter, resting on the top of the wall of the tower (fol. C. 2); a sextant of 4 feet radius for measuring distances (fol. E.); and a double arc for measuring smaller distances. Probably the last two instruments were removed or used on the open gallery when the triquetrum was erected, as the latter must have been large enough to fill the whole room, and, indeed, even in the southern observatory there cannot have been much elbow-room for the observers. In the northern observatory was also preserved an interesting astronomical relic, the triquetrum used by Copernicus, and made with his own hands.

By degrees, as Tycho's plans for collecting observations became extended and a greater number of young men desired to assist him, he felt the want of more instruments and of more observing rooms, in which several observers could be engaged at the same time without comparing notes. In 1584 he therefore built an observatory on a small hill about a hundred feet south of the south angle of the enclosure of Uraniborg, and slightly to the east. In