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Rh twenty-four hours, it was easy to make out the interval which passed between the culmination of the sun and a star by starting the time-keeper when the former passed the meridian, and letting it run until the latter passed, and then weighing the amount which had flowed out. Instead of mercury, Tycho also tried lead monoxide powder, and adds to his account of these experiments some remarks about Mercury and Saturn (lead), and their astrological relations, which naturally suggested themselves to his mind. But he does not seem to have used these clepsydræ except by way of experiment, and his methods of observing made him in most cases independent both of them and of the clocks. In addition to the altitudes (about which he justly remarks that they must not be taken too near the meridian, where they vary very slowly, nor near the horizon, where they are much affected by refraction), he observed hour-angles of the sun or standard stars with the armillæ to control the indications of his clocks, and his observations of the moon, comets, eclipses, &c., where accurate time determinations are indispensable, were thereby doubly valuable. Occasionally azimuths were also observed for the same purpose, the zero of the azimuth circle having been found by observing the east and west elongation of the Pole Star.

For observations of altitude Tycho also used a sextant of 5$1⁄2$ feet radius, turning on a vertical axis, with one end-radius kept horizontal by means of a plumb-line attached to the centre of the radius. We have already mentioned that