Spots and shadings have on various occasions been noticed on Venus, and though it is not easy to harmonise the various accounts, there seems no doubt of the reality of the facts, or that they must be ascribed to the existence of mountains. Schröter found very much the same state of things to exist on Venus that he found on Mercury, and putting together what he saw he arrived at the conclusion that Venus possesses mountains of considerable height, and that his observations must be taken to imply that the planet revolved on its axis in rather more than 23 hours. This conclusion as regards the planet’s axial rotation was not first arrived at by Schröter, for the two Cassinis, one about 1666, and the other about 1740, both ascribed to Venus a rotation period of about 23 hours, an evaluation which was fully confirmed by Di Vico at Rome between 1839 and 1841, and by Flammarion in 1894. What has been already said with respect to Mercury is true also of Venus, namely that it has been much neglected by modern observers; and accordingly an announcement made by Schiaparelli in 1890, that the rotation period of Venus is to be measured not by hours but by months, came upon the astronomical world as a startling revelation; but it is a revelation which has been keenly contested, and certainly awaits legal proof. Schiaparelli has not ventured to assert as he has done in the case of Mercury, that Venus’s rotation period is identical with the period of 7½ months in which it revolves round the Sun; he only claims this as a strong probability arising out of what he says he is certain of, namely that its period of rotation cannot be less than six months and may be as much as nine months. His assumption is that previous observers in endeavouring to ascertain Venus’s rotation period have used and relied upon evanescent shadings which probably were of atmospheric origin and scarcely recognisable from day to day, whereas he fixed his attention upon round defined white spots, which, whatever their origin, are so far permanent that their existence has been spoken of for two centuries. Miss Clarke thus puts the matter:—“His steady watch over them showed the invariability of their position with regard to the terminator; and this is as much as to say that the regions of day and night do not shift on the surface of the planet. In other words she keeps the same face always turned towards the Sun.” Various recent observations, some of them made with the express object of throwing light upon Schiaparelli’s conclusions, are strangely contradictory. Perrotin at Nice in 1890 thought his observations confirmed Schiaparelli’s; on the other hand Niesten at Brussels considered that numerous drawings of Venus made by himself and Stuyvaert between 1881 and 1890 harmonised well with Di Vico’s rotation period of 23h. 21m. 22s.; which Trouvelot in 1892 only wished to increase to about 24 hours.