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HISTORY OB PHYSICAL ASTRONOMY.

He is also the first astronomer who used clocks moved by weights for thepurpose of measuring time. These pieces of mechanism were introducedoriginally from eastern countries. Pacificus, Archdeacon of Verona, whoflourished in the ninth century, was the first European who constructed aclock with wheels. Others were soon fabricated in different countries ofEurope , but they were used only in the public edifices of great citiesbefore Waltherus applied them to the purposes of astronomy.

In consequence of the observations of Waltherus being the earliest ofmodern times, they were continually used at a subsequent period in recti-fying the elements of astronomical science by comparing them with similardeterminations of a more recent date. Lacaille has employed them forthis purpose in his researches on the solar theory. This is probably thelast important occasion on which the observations of Waltherus will beconsulted. In the present day it is found that the modern observationsmore than compensate by their superior accuracy for the shortness of thetime that has elapsed since they were made.

Copernicus , about the middle of the sixteenth century, had restored thetrue system of the world, but the epieyclical fabric still remained almostin its original state. To Tycho Brahe is due the immortal honour ofhaving executed the series of observations which led to the discovery ofthe real nature of the planetary movements. This illustrious individualexpended his whole private fortune, amounting to a hundred thousandcrowns, upon objects connected with the improvement of practical astro-nomy. His collection of instruments was one of the most magnificentwhich the world has ever seen. Waltherus, and the earlier astronomersof modern Europe , possessed only wooden appliances for observing thepositions of the celestial bodies. The instruments of Tycho Brahe weremainly constructed of metal. They were also larger and more accuratelydivided than any that had been hitherto applied to the purposes of astro-nomical observation. The ancient astronomer§§appear to have dividedtheir instruments only to every ten minutes. Some of Tycho Brahe 'swere actually divided to every minute; and by employing the method oftransversals, which came into use in his time, they were, in some instances,subdivided to every ten seconds *.

Not only were Tycho Brahe ’s instruments vastly superior to thosehitherto used by astronomers, but his methods of observation were alsosuch as to assure a greater degree of precision to the final results thanhad been hitherto attained. By taking into account the effect of refraction,he avoided, to a great extent, a source of error which had vitiated theobservations of all preceding astronomers. His method of determiningthe absolute right ascensions of the stars was also a vast improvement

* It may be remarked, however, that in his actual observations Tycho Brahe by no meansattained the degree of precision contemplated in the construction of some of his instru-ments ; for even although the divisions of the limb had been faultless in any case, theadvantage of the instrument in this respect would have been vitiated by the error com-mitted in the pointing of it to any celestial body, an error which it was impossible to avoidso long as observations were made with plain sights. With respect to the method of trans-versals, or, as it is more commonly termed in England, the diagonal scale, Tycho Brahe mentions ( Proggmnasmata, p. 671, edit. 1610) that he first became acquainted with itwhen he was a student at the university of Leipsic, having obtained a description of itfrom one of the professors named Homelius, but that he is ignorant of the real author ofthe invention. Thomas Digges {Alee sen Scales Mathematical, Capitulum Nonum ; Lon-dini, 1573) asserts that it was ihvented by Richard Chanzlcr, an English artist famous forhis skill in the construction of mathematical instruments, and that it had been long wellknown in England.