September 15, 1892] 



NA TURE 



471 



many curious features. The most prominent of them is 

 that this fine crescent is sometimes observed to extend to 

 a greater angular extent than i8o" (Fig. 12), 260° of the 

 limb of the planet having once been recorded. Some- 

 times, by adopting special precautions, the whole circum- 

 ference has been observed, the obscured disc being com- 

 pletely surrounded by a pale and thin luminous ring. 

 This, as M. Trouvelot says, is of very rare occurrence, 

 for it has happened that although the greatest precautions 

 have been taken, no trace of the planet could be found. 



Fig. 13. — Showing the bulging out of the 



1878. 



crescent as seen on February 24, 



When the crescent is extremely fine, great irregularities 

 have been noticed to mar the continuity of its curve ; they 

 differ also not only at different but at the same con- 

 junctions according as the planet is to the east or west of 

 the sun. 



Another fact that has been observed relates to the 

 bulging out of the planet (Figs. 13 and 14) at some parts 

 of its visible limb. This was especially noticed in the 

 month of February, 1878 ; while the crescent was being 



Fig. 14. — The crescent as ieen on February 26, 1878. 



looked at, the south-southeast portion seemed to suddenly 

 appear thicker than the remaining part. In fact, the 

 observer in the first instance thought it might have been 

 due to some optical delect in the instrument ; but sub- 

 sequent observation showed that this was not the case, 

 a real change of form having taken place. Two days 

 later this deformation was still more noticeable, the thick- 

 ness of the visible section being about double what it 

 would have been had it been in its normal condition. 

 NO. I 194, VOL. 46] 



Perhaps one of the most important points referred to 

 in this work is the determination of the period of rota- 

 tion by means of the spots. This question of rotation is 

 one that has baffled many observers, for the difficulty 

 that has presented itself lies not only in the proper 

 motions of the spots themselves, but in the identification 

 of the same spots after brief periods of time. Glancing 

 over some of the periods already obtained, we find that 

 Schroeter deduced from his observations a rotation of 

 about 24h., basing his value on the movement of a small 

 isolated spot situated in one of the horns. Fritsch's value 

 of 23h. 22m., and P. de Vico's 23h. 21m. 21s., are both 

 also of about the same length. From observations by 

 D. Cassini and F. Bianchini, we have a very wide devia- 

 tion, the periods of rotation being reckoned in days, the 

 former arriving at a value of 23 days, and the latter at a 

 somewhat larger one of 24 days 8 hours. Coming now 

 to Schiaparelli's value of 225 days, we have here altogether 

 a new departure, the planet rotating on its axis in the 

 same time as it revolves round the sun. 



With such values as these it will be at once seen that 

 there is something radically wrong with the spots or their 

 positions on the planet's surface ; in some cases, of 

 course, there might have been instances of mistaken iden- 

 tity, but with such an observer as Schiaparelli, who very 

 definitely settles upon a 225 daily period obtained from 

 direct observation, it is hard to conceive that any such 

 sources of error would not have been remarked. 



The observations which we have now before us bear 

 out Schroeter's view of a short rotation, Prof. Trouvelot 

 telling us that they were made during the years 1876-78 

 under exceptionally good conditions. One very interest- 

 ing point which is of great importance is the fact that 

 these observations were made at the same period, " sou- 

 vent dans la mcme journde, sous un ciel egalement pro- 

 pice et precisement sur la meme point de la planete." 



The value nearest to 24 hours that Prof Trouvelot ob- 

 tained was 23h. 49m. 28s., and in giving this period he re- 

 marks that it is founded on the supposition that the spot 

 had no prooer motion. In referring to the period deduced by 

 Schiapareili he says, " La cause probable de I'erreur de 

 M. Schiaparelli semble rdsulter de ce fait que les taches 

 h et k, qui ont servi de base a ses conclusions, faisaient 

 partie de la tache polaire meridionale qui, etant situ^e 

 centralement sur I'axe de rotation de la planete, semble 

 rester stationnaire, comme cela se voit sur la tache polaire 

 de Mars, quand elle se trouve reduite a de faibles dimen- 

 sions." Taking into account many of the general features 

 visible on the planet's surface, such as the rapid deforma- 

 tions of the horns and of the terminator, all these point to 

 short periods of rotation, which, as Prof. Trouvelot points 

 out, is "inconciliable avec la periode de rotation, si lente 

 et si inattendue, ddduite par leminent astronome de 

 Milan." 



In concluding our remarks we cannot help mentioning 

 the very complete way in which Prof. Trouvelot has 

 taken into account the prior work in this interesting field 

 of inquiry. W. J, L. 



NOTES. 



The Iron and Steel Institute will meet at Liverpool from 

 Tuesday, September 20, to Friday, the 23rd. Sir Frederick 

 Abel will preside. The following papers will probably be lead 

 and discussed : — (Tuesday) on the condensation of ammonia 

 from blast furnaces, by Sir L. Bell, F.R.S. ; on alloys of chrome 

 and iron, by R. A. Hadfield ; on the Liverpool overhead 

 railway, by J. H. Grealhead : (Wednesday) on the engineering 

 laboratories in Liverpool, by Prof. H. S. liele-Shaw ; on the 

 Siemens-Martin process at Witkowitz, Austria, by P. 

 Kupelwieser ; on failures in the necks of chilled rolls, by C. A. 

 Winder: (Thursday) on a new process for the elimination of 



