April 27, 1876] 



NA TURE 



51, 



the greater number of revolutions will the samara perform ere 

 reaching the ground, and the longer consequently will be the 

 path through which its friction is exercised. 



In seeking a model for a screw-propeller, we must remember 

 that the pitch should vary with the velocity of propulsion. 



London, April 15 Wm McLaurin 



OUR ASTRONOMICAL COLUMN 



The Rotation of Ven'us.— It was Jean Dominique 

 Cassini (Cassini I. as he has frequently been designated) 

 who during his residence in Italy, made the first serious 

 attempt to ascertain the lime of rotation of this planet 

 and the position of the axis. His obser\'ations with one 

 of Campani's long telescopes appear to have been com- 

 menced about the middle of the seventeenth century, as 

 related in the Journal des Savatis, 1667, Dec. 12, but it 

 was not until the evening of Oct. 14, 1666, that he per- 

 ceived any spot of sufficiently definite aspect to be of 

 service for the purpose in view. It is described as " Une 

 partie claire situee proche de la section, et fort eloignee 

 du centre de cette planeie vers le septentrion." At the 

 same time several dusky spots were noted. These obser- 

 vations were continued till June i667,but Cassini expressed 

 himself very cautiously with regard to the inferences to 

 be drawn from them. They appeared to indicate a 

 return of the bright spots to the same position upon the 

 disc at intervals of about twenty-three hours, but from 

 the short time that the spots could be followed Cassini 

 was unable to decide whether the appearances were to be 

 attributed to an axial rotation or to a libration. " De 

 dire maintenant," he says, " suppose que ce soit toujours 

 la meme partie luisante, si ce mouvement se fait par une 

 libration, c'est ce que je n'oserais encore assurer, parce 

 que je n'ai pas pu voir la continuite de ce mouvement dans 

 une grande partie de Tare, comme dans les autres planetes, 

 et par cette meme raison cela sera toujours tres-difficile 

 k determiner." 



In 1726 Bianchini, domestic prelate of the Pope, ob- 

 serving at Rome, with glasses, also by Campani, of 70 to 

 ICO Roman palms in focus, remarked on Feb. 9 several 

 spots which he continued to observe with the view to 

 determine the time of rotation. His observations were pub- 

 lished in " Hesperi et Phosphori nova Phenomena," 1728, 

 and he considered them to show a period of rotation of 24 

 days 8 hours, the North pole of Venus being directed to 

 longitude 320", with an inclination of 15° only to the plane 

 of the ecliptic. Bianchini's observations appear to have 

 been made under very unfavourable conditions, whereby 

 he was prevented from following the spots in a continuous 

 manner. They were discussed at length by Jacques 

 Cassini, the son of Jean Dominique, in " Elemens 

 d'Astronomie" (1740), who arrived at the conclusion that 

 a rotation of 23 hours 20 minutes would represent equally 

 well his father's observations and those of Bianchini, 

 while if the rotation assigned by the latter was admitted, 

 it would be necessary to reject entirely the observations 

 of the elder Cassini, " comme n'etant qu'une apparence 

 trompeuse." 



Jacques Cassini mentions that after Bianchini had 

 communicated to him the observations at Rome, he 

 made attempts to discern the spots upon Venus at Paris. 

 He examined the planet on a great number of occasions 

 with a glass of 114 feet focus, one of the best produced 

 by Hartsoeker, and also with one of Campani's, of 120 

 Roman palms' focus, which had been tried by Bianchini 

 and considered excellent, but with all the precautions 

 taken neither he nor Maraldi could perceive any distinct 

 spot upon the planet's disc. 



Schroeter, in 1789, examining Venus with a 7-feet re- 

 flector, discerned a bright spot in the dark hemisphere, 

 and by following the appearance of this object, inferred 

 that the planet rotated in 23h. 21m. 19s., thus supporting 

 ihe result obtained by Jacques Cassini from his fathers 



observations. Schroeter's observations appear in D. J. H. 

 Schroeter, " Cythereographische Fragmente, cd;r Beo- 

 bachtungen iiber sehr Betrachtlichen geberge und rotation 

 der Venus," Erfurt, 1792; and in " Aphroditographische 

 Fragmente, &c.," Helmstadt, 1796. In an appendix to 

 the latter work, noticed by Zach in " Monatliche Corre- 

 spondenz," xxv. p. 366, it is stated that obser\'ations ot 

 " atmospheric sf>ots," and of the horns, with eight deter- 

 minations of " a definite point upon the surface," give 

 for the final value of the rotation-period of Venus, 

 23h. 2im. 7'98s. 



De Vico's observations and investigations bearing upon 

 the time of rotation and the position of the axis are pub- 

 lished in "Memoriadella Specola ... in CollegioRomano," 

 1840-41, p. 32, and in the succeeding part of the same for 

 1842, p. 29. The period of rotation assigned from these 

 observations, which were made with the Cauchoix refrac- 

 tor of the Roman College, is 23h. 21m. 21 '935. (sidereal 

 time). The longitude of the ascending node of the 

 equator of \'enu3 upon her orbit is fixed to 56^ 30', and 

 the inclination thereto 53° 11', while for the same elements 

 referred to the ecliptic we have 57° 19' and 49" 57'. There 

 is some error of the press or of calculation here which it 

 is not easy to rectify. In a note to Secchi's Life of De 

 Vico, '■' Memorie dell Osservatorio ... in CoUegio Romano," 

 Anno 1850, p. 140, the inclination of the equator of Venus 

 to the echptic is given, 53° 11' 26", and the longitude of 

 the ascending node of the equator upon the echptic for 

 1841, 57= 19' 18". 



Notwithstanding the near agreement of De Vico's 

 period of rotation with that assigned by Schroeter, it 

 must be admitted that further investigation is very essen- 

 tial before we can consider the period established. There 

 are so many negative observations upon record and these 

 made under circumstances at least as favourable as those 

 upon which the rotation of the planet has been supposed 

 to be fixed, that there is ample justification for doubt in 

 the matter. 



We hear from more than one correspondent that dusky 

 spots have been suspected upon the disc of Venus, within 

 the last few weeks ; if there be no illusion, the present 

 may prove a favourable opportunity for attempting a new 

 determination of the rotation-period, and this considera- 

 tion has suggested the above outline of the actual state of 

 our knowledge upon the subject. 



MIXOR Plaxets.— By a note from Herr Palisa it 

 would appear that the small planet observed at Pola on 

 November 22 and 23 was not, as supposed at the time, 

 identical with the one he had detected on November 8. 

 No. 155 is therefore lost or in similar predicament to the 

 planet observed by Watson, 1S73, July 29. 



M. Leverrier's Bulletin International, of April 22, 

 announces the discovery of another small planet at 

 Paris, by M. Prosper Henry, during the previous night, 

 in R.A. I4h. 9m. 58s., N.P.D., 102° 18'. 



PROF. FLOWER'S HUNTERIAN LECTURES 

 ON THE RELA TION OF EXTINCT TO EXIST- 

 ING MAMMALIA ^ 



VI n. 



T^HE existing famihes of the Carnivora, spoken of in 

 -*• the last lecture, do not appear to have been dis- 

 tinctly differentiated in the Eocene period, at all events 

 not till towards its close, but the order was represented by 

 other and verj- singular forms, the systematic position 

 of which is not easy to determine. The earliest in point 

 of time is Arctocyon primcsvus, from the lowest Eocene of 

 La Fere, Aisne, France, an animal nearly as large as a 

 wolf, with a long tail, and heavy, strong limb bones, and 



' Abstract of a course of lectures delivered at the Royal College of Sur- 

 geons " On the Relation of Extinct to Existing Mammalia, with Special 

 Reference to the Denrative Hypothesis," in conclusion of the course of 1875. 

 (See Reporuia Nature for that year.) Coritinued ftx)m p. 48S. 



