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HISTORY OK PHYSICAL ASTRONOMY.

It appears from the work of Tycho Brahe, above cited, that ThaddeusHagecius, another contemporary astronomer who wrote upon the newstar, advanced a step further than the Landgrave ; for he shewed that boththe declination and right ascension of a star might be determined byobserving its altitude on the meridian, and noting the time of observation.Tycho has given, as an example of this method, Hagecius 's determinationof the place of the new star. He takes occasion, at the same time, to pointout the various defects of the method, and it must be admitted that, in theexisting state of practical astronomy, his objections were not without weight.If the time was to be indicated by clocks, it would be impossible, he re-marked, to avoid errors of serious magnitude, as he had already shewn ona former occasion. If the observed altitude of a star was employed forthat purpose, the chance of error would be equally great, for neither couldthe absolute place of the star be ascertained beforehand with sufficientprecision, nor could its altitude, corresponding to each observation, besatisfactorily determined. Moreover, when a star is approaching themeridian, its altitude increases very slowly, so that it is exceedingly dif-ficult to assign the precise instant when it is actually upon the meridian.This source of error might be avoided by observing the star near thehorizon, where it rises with great rapidity ; but, on the other hand, theuncertain effect of refraction at low altitudes would tend to vitiate theobservation *.

Tycho Brahe , for the above reasons, did not deem it safe to employ theelement of time in determining the apparent place of a celestial body.In fact, the observations of the Landgrave on the new star, to whichallusion has already been made, supplied him with a practical proof ofthe justness of his views on this subject. In each of these observationsthe altitude, the azimuth, and the time were different; but as the posi-tion of the star in the celestial sphere was found to be invariable, itfollowed, as a necessary consequence, that each combination of these threedata ought to assign to the star the same values of declination and rightascension. Tycho calculated these elements by means of the Landgrave ’sobservations; but he found that although the declinations exhibited apretty satisfactory agreement with each other, the right ascensions dif-fered, in some instances, to the extent of 2° of space f. This may not bean improper place for a few words on the construction of the clocks ofthose days. They consisted essentially of a vertical wheel, which wasmade to revolve with a slow motion by the gradual descent of a weight.The balance was a happy contrivance, devised for the purpose of con-stantly checking by its inertia, the descent of the weight, and therebyrendering the motion of the wheel more uniform. It consisted of a cross-bar, with two weights at its extremities, capable of turning freely in ahorizontal direction, upon a vertical axis, which was connected with thewheel by means of two pallets. The distance between the pallets wasequal to the diameter of the wheel, and they were so disposed on thevertical axis as to engage alternately the highest and lowest tooth of thewheel. When the highest tooth engaged, the upper pallet, the balancewas pushed round, and its resistance checked the descent of the weight.As soon as this pallet became freed from the wheel, the lowest toothengaged the other pallet, and the balance was then pushed in the oppo-site direction. This alternate movement of the balance continually

Progymnasmata, p. 523.

f Ibid., p. 502.