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HISTORY OF PHYSICAL ASTRONOMY.

As in the case of Mercury, the intense lustre of this beautiful planethas proved a serious obstacle to astronomers, in their attempts to arrive atsome knowledge respecting its physical constitution. In the years 1666-7Cassini, while still residing under the clear sky of Italy , devoted his atten-tion to a searching examination of the planet, with the view of detecting pe-riodic changes in its appearance which might indicate a rotation on an axis.On the 21st April, 1667, he at length discovered a bright spot, which,when watched for some time, was found to return to the same position onthe disk of the planet at successive intervals of about 23 hours. This wouldindicate a rotation of the planet agreeing very nearly with the diurnal mo-tion of the earth. Cassini, however, was unable to follow the spot through-out an extent of its course sufficiently large to enable him to ascertainpositively whether its change of position arose merely from a libratory mo-tion of the planet, or from a complete rotation on an axis. Upon a subse-quent occasion, during his residence in France , he resumed the examinationof the planet, with the view of perfecting his previous researches, but al-though he strove, with great perseverance, to detect the spot on its diskwhich was visible in Italy , all his efforts were fruitless, In 1726 Bianchiui,the domestic prelate of the Pope, commenced an examination of Venus atRome , with a powerful telescope constructed by the celebrated Campani, andsoon succeeded in detecting several dark spots on the disk of the planet, themotion of which he continued for some time to watch with great attention.The conclusion he arrived at was somewhat anomalous. He announcedthat the changes in the positions of the spots could not be satisfactorilyaccounted for without supposing a period of rotation amounting to 24 a 8 h .The researches of astronomers on the rotation of the planet formed thesubject of a critical discussion by J. Cassini. He remarked that the con-tinuity of Bianchini’s observations was interrupted by the interpositionof the Barbarini Palace, which every night concealed the planet from himfor some time, Adopting a hypothesis founded on this chasm in the ob-servations, he shewed that if the period of rotation be supposed equal to23 h 20 m , the observations of his father as well as those of Bianchini maybe equally well satisfied; but he remarked that if the period of 24 d 8 h beadmitted, his father’s observations must be rejected as totally worthless*.

The value assigned by Cassini to the period of the rotation of Venus has re-ceived a complete confirmation from the researches of modern astronomers.In 1789 Schroeter, having undertaken a careful examination of the planet‘with a seven-feet reflector, discovered a luminous point in the dark hemi-sphere, a little beyond the southern horn, indicating the existence of ahigh mountain. By continuing for some time to direct his attention tothe periodic changes of this object, he was finally enabled to deduce fromthem the conclusion, that the planet performs a complete rotation on itsaxis in a period amounting to 23 h 21 m 19 s . This result agrees with thatrecently arrived at by the late Sig. De Vico and his associates, ofthe Observatory of the Roman College . In 1839 they commenceda series of observations of Venus with a magnificent refractor by Cau-choix. In order more effectually to obviate the inconvenience arisingfrom the intense lustre of the planet, the examination was conductedduring the day timef. The ancient spots observed by Bianchini werespeedily re-discovered, and were found to possess the precise forms assigned

* Siemens d’Astronomie , p. 526.

f A similar mode of observing the planet was employed by La Hire. See Mem.Acad, des Sciences, tome x., p. 20.