HISTORY OF PHYSICAL ASTRONOMY.
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to them by that astronomer. The final conclusion was, that the planetperforms a complete rotation on its axis, in a period agreeing almost ex-actly with that which Schroeter had deduced from his observations.
The various astronomers who have determined the rotation of Venus agree in estimating the inclination of the equator to the plane ofthe ecliptic at 75°. No trace of ellipticity has been discovered in theplanet; nor will this appear surprising when the probable smallnessof its amount is considered. If the equatorial axis exceeded the polar byonly siUth of its own length, as in the case of the earth, the differenceof the two axes would subtend an angle of only a tenth of a second at themean distance of the planet from the earth. It is obvious that themost delicate micrometrical measurements are incapable of establishingthe existence of so small a quantity. That the ellipticity of the planetdoes not differ much from that of the earth, is probable from the circum-stance that both bodies are nearly of the same size, and are enduedwith nearly equal velocities of rotation.
According to Schroeter there exist mountains of immense height on thesurface of Venus . The most considerable of these elevations amounts tojof the radius of the planet. Chimborazo, in South America , whichwas long supposed to be the highest mountain on the surface of the earth,attains an elevation equal only to nfai °f the terrestrial radius. Thesame astronomer remarks that Venus, as well as Mercury and the Earth ,have the highest mountains situate in their southern hemispheres. Theseresults can only be admitted with a certain degree of reserve, when theextreme delicacy of the observations by means of which they are arrived at,is taken into account. Even the great Herschel was unable to deduceany trustworthy results of this nature from his observations of the planet.The remark of Schroeter, that the three planets nearest the sun have thehighest mountains situate in their southern hemispheres, is at least nolonger applicable to the earth, since the Himalayah range of mountainshas been found to he the most elevated region on its surface.
Various circumstances concur to prove that Venus is surrounded byan atmosphere of considerable density. During the transits of theplanet over the sun’s disk in 1761 and 1769, the planet was observed,by several astronomers, to be surrounded by a faint ring of light similarto the appearance which would be occasioned by the passage of the solarrays through a circumambient fluid. This indication of the existence ofan atmosphere is confirmed by the observations of Schroeter, who dis-covered what appeared to him to be a faint crepuscular light, extendingbeyond the cusps of the planet into the dark hemisphere *. From micro-metrical measures of the space over which this light was diffused, he con-cluded that the horizontal refraction at the surface of the planet amountsto 30' 34", a quantity agreeing very nearly with the horizontal refractionof the terrestrial atmosphere f.
When the planet Mars is observed with a telescope of ordinary power,its surface appears to be diversified with spots of an irregular cha-racter, which have conducted astronomers to some interesting conclusionsrespecting its physical constitution. These appearances were first noticedby Fontana, a Neapolitan astronomer. In 1636 he observed a spot onthe disk of the planet, which subsequently re-appeared in 1638. Fromthe changes which it seemed to undergo, he suspected that the planet w'as
* Phil. Trans., 1792.
f The interesting fact of the existence of a twilight in Venus has been confirmed bythe observations of Sir William Herschel . (Phil. Trans. 1793, p. 214.)