No^-EMBEE 1, 1897.] 



KNOWLEDGE. 



259 



petually dark and cold. Librations, however, would 

 materially vary the bounding line between Ught and shade. 



Whilst Cassini, Bianchini, Schroeter, and De Vice 

 affirm that they had observed certain displacements of 

 spots and certain periodical phenomena, the illustrious 

 Director of the Milan Observatory assures us that the 

 divers markings, whether light or dark, that are sometimes 

 perceptible on the planet Venus, do not move. His main 

 observations extend from the 5th of November, 1877, to 

 the 7th of February, 1878, and comprise about one 

 hundred sketches of the planet, many of which were taken 

 on the same day. At first, viz., between the 5th of 

 November and the 7th of December, the observer "only 

 perceived confused shadows comparable to those of 

 Bianchini and De Vico, and which could by no means 

 serve for a proper estimate of the time of rotation." On 

 the 8th of December the most certain spot was a triangular 

 shade located somewhere near the centre of the visible 

 semi-disc. Ou the 9th the spot was again seen, and 

 again on the 10th. The apex of this great triangular 

 patch was directed towards the north. Above — i.e., at the 

 southern horn — there might be seen white spots, and these 

 were again visible on the following days. We here 

 reproduce the three illustrations that were published in 

 this connection by M. Schiaparelli. In the first, the 

 region extending from m to h was the darkest, and this 

 line of direction )n h will also be found in the two other 

 sketches. This dark traU or streak h m was, moreover, seen 

 also by Mr. Holden, at Mount Hamilton, and M. Niesten, 

 at Brussels. 



It is chiefly the immobility of this mark for more than a 

 month that convinced M. Schiaparelli of the impossibility 

 of a rapid diurnal rotation. While it was kept under 

 observation for several hours each day, for several days 

 and weeks, it did not alter its position, though it did 

 change in tone and extent. 



Fig. C. — Eepresentatiou3 of the Planet Venus by M, Schiaparelli, 

 1877. (V Astronomie, 1890, p. 418.) 



In 1895 the same writer reverted to the subject in a I 

 letter, quoted below, which he addressed to M. Terby on 

 the 81st of July, in reference to the observations of Herr 

 Leo Brenner-^which gentleman, however, so far as he 

 himself is concerned, regards these very same observations 

 as conclusively showing that a rapid rotatory movement is 

 completed in 23h. 57m. 7-5'159s.* 



"At all events, the present spot of Venus does really 

 and truly exist, and it reflects credit upon Herr Brenner to 



* Bulletin de la Societe Astronomique de France, 1895, pp. 275 

 and 336. 



have called attention to it ; for the study of that spot has 

 put the final seal of certainty on the rotation of 224-7 

 days ; whereas, previously, as you well know, the demon- 

 stration of that fact had rested upon too small a number 

 of observations. 



"I am sending you herewith a copy of one of my 

 drawings taken with the eighteen-inch glass, which I con- 

 sider the best of all, and which represents the state of the 

 planet during the last few days of July. I have several 

 times compared it with the planet in every detail, andi 

 am satisfied with it on the whole, taking into account that 

 the shades in question are extremely difficult objects. 

 Only the large spot near the southern horn is visible 

 with comparative ease, and my colleague. Dr. Rajna, has 

 satisfied himself of its existence. It is more readily seen 

 during the day, when near culmination, especially when 

 the sky is hazy and the planet comparatively dull. This 

 spot, with the shadows connected with it, is undoubtedly 

 the same that I observed in 1877 (apart from certain 

 minute details). On looking through my old observations, 

 I find that I have sketched the same objects in February 

 and March, 1881. There is evidently a certain stability 

 in the variations, or, at any rate, a periodical recurrence of 

 analogous conditions. The somewhat bold conjectures 

 on the subject to which I gave expression in 1890, are 

 thus verified in aa satisfactory a manner as could well be 

 expected. 



" However that may be, it will now require a good deal 

 of boldness to return to the 23h. 20m. rotation period 

 again. The spot is there — perfectly motionless at all 

 hours of the day ; at least, since the 3rd of July." 



We reproduce this drawing from the original published 

 in the Bulletin de VAcademie de Behjique. 



Thus, according to M. Schiaparelli, it is this spot that 

 " has put the final seal of certainty " to the rotation of two 

 hundred and twenty-four days, the demonstration of which 

 had rested upon " too small a number of observations." 

 Look also at this phrase : " There is evidently a cer- 

 tain stability in the variations, or, at any rate, a 

 lieriiidical recurrence of analogous conditions." 



Now there is not the slightest reason for supposing 

 that this spot, any more than the others, forms part 

 of the surface of the planet. If it were a fixed spot on 

 the globe, the careful observations made by Bianchini 

 and De Vico, under the purest sky and by means of 

 excellent objectives, should have demonstrated its 

 presence as well as the more recent investigations to 

 which I am now coming. 



At the same time as M. Schiaparelli — in December, 

 1877 — M. Trouvelot made identical observations at 

 Cambridge, but concluded from them a rapid rotation. 

 Indeed, in the markings which the learned chief of 

 the Milan Observatory held to be " stable," he (Trou- 

 velot) notices as many as seventeen changes,! showing 

 clearly that those markings are atmospheric, and not 

 geographical, forms. Once again, if these spots were 

 permanent, as are those of Mars, they would be found 

 in the different series of drawings. Again, any spots 

 on the surface cannot but be invisible, owing to atmo- 

 spheric absorption, which is not, and cannot be, disputed 

 by any astronomer. Without knowing it, M. Schiaparelli 

 has given us the true word which destroys his own 

 theory. We have there "a periodical recurrence " of ana- 

 logous conditions. How often do we not find, upon the 

 earth, that the same hour brings back the same atmo- 

 spheric state ! Seen from Mars, at an obliquity, and 



* Bulletin de I'AcadJmie de Belgiqueioi August, 1895. 



+ Bulletin de la SociM Astronomique de France, 1892, p. 134. 



