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324 adds that this clock was a very good one, and indicated correctly the interval between two successive noons; but all the same he must have seen how unreliable it was, for though he used the clock during the lunar eclipse in 1487, he at the same time measured some altitudes.

In Tycho Brahe's observatory the clocks never played an important part. Though he possessed three or four clocks, he does not anywhere describe them in detail, while he in several places remarks that he did not depend on them, as their rate varied considerably even during short intervals, which he attributed to atmospherical changes (although he kept them in heated rooms in winter), as well as to imperfections in the wheels. At the side of the mural quadrant he had placed two clocks, indicating both minutes and seconds, in order that one might control the other, and in the southern observatory was a large clock (horologium majus) with all the wheels of brass. Whether Bürgi, during Tycho's residence at Prague, supplied him with a pendulum clock, as stated by a later writer, must remain very doubtful, but that Tycho did not possess such a clock at Uraniborg seems certain, as he would not have neglected to describe so important an addition to his stock of instruments. As he found the clocks so uncertain, Tycho also tried time-keepers similar to the clepsydræ of the ancients, which measured time by a quantity of mercury flowing out through a small hole in the bottom of a vessel, in which the mercury was kept at a constant height, in order that the outflow might not vary with the varying weight of the mercury. By ascertaining the quantity of mercury which flowed out in