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HISTORY OF PHYSICAL ASTRONOMY.

-which the celestial bodies might be observed from east to west throughoutthe whole of their diurnal courses *. Such a mode of observation mighthave been very serviceable for the discovery of satellites, and for the ex-amination of the physical constitution of the planets, bnt in so far as thedeterminations of the positions of the celestial bodies was concerned, thetendency of the age was towards observations made exclusively in theplane of the meridian with fixed instruments. The alteration proposed byCassini was, in fact, such a one as Tycho Brahe would have recommendedfor the purpose of making ordinary observations of the celestial bodies withhis equatorial armillse. Another alteration suggested by him was theadmission of the solar light through an aperture, so as to trace upon thefloor the daily course of the sun throughout the year. By this con-trivance he contemplated the formation of a vast sun-dial: lie also ex-pected to derive from such observations a more accurate knowledge of thevariations of refraction. It is not difficult to perceive that such an alter-ation, if it had been carried into effect, would have been in reality a stepbackwards rather than a positive improvement. The remarkable successwhich attended Cassini’s early efforts of a similar kind at Bologna, seemsto have given a bias to his views on practical astronomy, from which hewas unable completely to emancipate his mind throughout the wholecourse of his life. It does not seem to have occurred to him that the use ofthe telescope and the pendulum, introduced into the practice of observationa degree of delicacy and precision, which left all such clumsy contrivancesimmeasurably behind. It must be acknowledged, therefore, that notwith-standing the distinguished talents of Cassini, and the many brilliant dis-coveries which he made in celestial physics, the position which he occupiedin France at the time of the establishment of the Royal Observatory ofParis, was unfavourable to the advancement of practical astronomy in thatcountry.

Picard commenced his observations at the Royal Observatory of Parison the 9th of July, 1673. This admirable astronomer was one of thefirst to perceive that the improvement effected by Huyghens in the con-struction of clocks completely removed the difficulty experienced byTycho Brahe and his contemporaries in their attempts to employ the ele-ment of time in astronomical observations. It was now clearly seen thatby determining the altitudes of the celestial bodies as they passed themeridian, and noting the times of observation, their declinations andright ascensions would be obtained with a degree of accuracy commensu-rate to the existing state of theoretical astronomy. The simplicity of thismethod gave it an incalculable advantage, as soon as it was found to bepracticable, over any other that had been hitherto employed for a similarpurpose. With the view of carrying it into effect, Picard solicited theestablishment of a quadrant in the plane of the meridian, but althoughhe continued to labour several years at the observatory, the object of hisrequest was not accorded to him. Not having such an instrument avail-able to him, he determined the instant of the passage of a celestial bodyacross the meridian, by noting the times at which the body attainedthe same altitude on each side of that circle f. It is manifest that the

* “ Memoires pour servir a l’Histoire des Sciences, ” p. 294.

f Sometimes the same object was effected by means of observations made with a tele-scope attached to a wall whose face was in the plane of the meridian. This instrument,termed la lunette murale, was also occasionally employed in determining the apparentaltitudes of the stars included within a small range of observation.