PHYSICAL AND MATHEMATICAL SCIENCES. 139

him the donation of the late king. Thus he wasdriven to seek refuge in other countries: and, underthe patronage of the emperor Rodolph II. (who ap-pears to have valued him chiefly from the assistance hislabours would give to the astrological art), he renewedhis observations at Prague . But his health and spiritswere broken ; and he at length sunk under a lingeringdisease, in 1601.

In reviewing his scientific labours, we find that inthe commencement of them, Tycho felt most sensiblyas the capital deficiency in the observatory, the want ofaccurate measures of time: he successively tried anumber of contrivances, and at length the imperfectclocks then constructed, but without any material suc-cess. Before the perfect construction of clocks, astro-nomers were necessitated to adopt various methods offinding the right ascensions, and thence the longitudesof the stars, by observations on their distances from thesun, which was effected by an intermediate comparisonwith the moon ■ or, according to Tycho’s suggestion,with Venus. He endeavoured to improve upon theaccuracy of preceding catalogues, by numerous deter-minations of this kind. He also invented the simplemethod of finding the latitude of a place by observingthe meridian altitudes of a circumpolar star when aboveand below the pole. He investigated very fully thecorrection due to atmospheric refraction ; though heascribed it rather to vapours than to the atmosphereitself, and entirely exploded the trepidation which hadso long disgraced the astronomical tables.

In comparing the positions given by Hipparchus withthose in his own catalogue of the fixed stars, Tychowas led to the important discovery of the slow dimi-nution of the obliquity of the earth’s axis, which is sogradual, that it cannot be detected but by comparisonof observations at very distant intervals: its existencewas, therefore, long disputed ; but modern observationshave established it beyond question, and it has beenshown to be a consequence of gravitation.