534 



NATURE 



{Oct. 12, 1876 



in a group of facute near the north-east Hmb in the place 

 indicated by Herr Weber's observation. The position of 

 the spot has now been measured on the two photographs, 

 which were taken at 2ih. 46m. 35s. and 22h. im. 4s. 

 Greenwich mean time respectively, and the following are 

 the means of the two sets of results which agree very 

 closely : — 



1876, April 3d, 2ih. 54m. 



Distance from sun's centre along arc of 



parallel 



Diff. of R.A. (Spot - ©) 



Diff. of N.P.D. (Spot - ©) 



Distance from sun's centre 



Distance from N.E, limb 



Dlamtter of spot 



788" 



+ 52^-3 



- 2i8"-5 



817'' 



145" 



4" 



As Herr Weber's observation was made at 4h. 25m. 

 Berlin mean time, or 3h. 31m. Greenwich mean time, the 

 sun's rotation in the interval — sh. 37m.^ — would have 

 carried the spot to a distance of about 163" from the 

 limb, as appears from a rough computation, and thus the 

 position would agree tolerably well with that given by 

 Herr Weber. There can be no question that the spot on 

 the Greenwich photographs, which is the same as that 

 observed by M. Ventosa, is an ordinary sun-spot without 

 penumbra, and not an intra-mercurial planet. 



Royal Observatory, Greenwich, October 4 



CA UTIONS AS TO INTRA-MERCURIAL 

 OBSERVATIONS 



AT the Paris Academy on the 2nd instant. Dr. Janssen 

 read a paper containing some very timely cautions 

 as to the observation of the transit of intra-Mercurial 

 bodies across the sun. He maintains that we have the 

 means of investigating the problem which at present is in- 

 teresting astronomers of a most satisfactory kind and lead- 

 ing to a certain and rational result. The first of these means 

 is the knowledge we now possess of the solar envelope, and 

 the second is photography. A criterion of a true transit is 

 that the spot be well rounded against the solar disc, that 

 it have a rapid displacement on the surface of the disc, a 

 motion quite different from the apparent motion of solar 

 spots. These requirements would eliminate a great number 

 of doubtful observations, and even then the transit might 

 not be a real one. Many solar spots are distinctly rounded, 

 but then error is apt to creep in in the observation of 

 the proper movement, especially when the observation is 

 made with a telescope having no equatorial mounting, the 

 diurnal motion making the spot appear to be constantly 

 changing place. The rapid disappearance of a spot is no 

 proof that it is outside the sun ; at the minimum period 

 spots have a tendency to dissolve rapidly. It follows that 

 the isolated observations made by persons who have no 

 thorough knowledge, or who have not suitable instru- 

 ments, are comparatively valueless. While giving the 

 highest place to photography, Dr. Janssen thinks tele- 

 scopic observations of so great importance that he gives 

 some hints for the guidance of observers. 



There are circumstances connected with the constitu- 

 tion of the photosphere which may afford guidance even 

 in fugitive observations. Briefly, as a solar spot is a phe- 

 nomenon of the photosphere, a disturbing phenomenon 

 at the highest point of the region where it is produced, it 

 follows that the ordinary aspect of the photosphere is modi- 

 fied all round it. Moreover, if the spot is sufficiently distant 

 from the centre of the disc, it ought to present the 

 perspective effects of an object placed upon the vanishing 

 surface of a globe. Finally the region of the sun where 

 the spot appears ought to be attended to, to discover its 

 solar latitude, since we know that the spots are located in 

 two main regions, to the north and to the south of the 

 sun's equator. More valuable still is the following test. 

 It is evident that a moving body interposed between our 

 eye and the solar surface ought to produce a succession 



of eclipses of the granulations covering that surface ; to i 

 cover successively those towards which it moves and ^ 

 uncover those on the opposite side. This phenomenon 

 of emersion and immersion is the most decisive of all 

 tests of the value of a brief observation ; it requires, how- 

 ever, a good instrument of considerable power. Dr. 

 Janssen advises moreover that the regions around the 

 sun's disc to three or four minutes angular distance should 

 be explored with the greatest care ; as at that distance the 

 coronal atmosphere is bright enough for a body of a 

 fraction of a minute in diameter to give a visible eclipse. 

 A trustworthy observation of a body seen either entering 

 or leaving the sun's disc under such circumstances, is 

 of the very highest value ; moreover the field of obser- 

 vation is thus greatly increased. But eye observations 

 of the sun must at best be but isolated, and photography 

 furnishes the only sure method of unerring, precise, 

 authentic observation, surpassing in value that of the 

 ablest astronomer. 



The question of intra-Mercurial bodies shows once more 

 the immense importance of obtaining uninterrupted inter- 

 national observation of the sun's face. Hence the value 

 of a mechanical photographic revolver that would, every 

 hour, say, photograph the sun, without requiring the 

 interference of any one. A number of these distributed 

 over the globe would, in a few years, give us such a 

 knowledge of the sun's surface as it would be impossible 

 to obtain under any other circumstances. 



RUSSIAN EXPLORATION IN ASIA DURING 

 THE PAST SUMMER 



THE following information as to the different scientific 

 expeditions sent during the past summer by the 

 Russian learned societies for the exploration of various 

 parts of Russia and of the adjacent territories will pro- ^ 

 bably be of interest. We begin with Central Asia, leaving 

 for another paper the report upon the proceedings of the 

 expeditions to the Obi and Jenissei. 



M. Prshevalsky has left Omsk, and we have already 

 given some account of the scientific staff of the expedition 

 and the route he proposes to follow. 



M. Severtsoff, as reported by the Turkestatiskija 

 Vedomosti, was to begin his travels in the Fergana dis- 

 trict and in the adjoining hilly tracts during this autumn. 

 He will be accompanied by M. Sharz, astronomer, M. 

 Mushketoff, mining engineer, M. Smirnoff, botanist, M. 

 Skvortsoff, zoologist, one topographer, and six Cossacks. 

 During next summer he proposes to explore the Alai 

 and the mountains south of Kokan, and to penetrate 

 about the autumn into the Pamir, reaching here the route 

 followed by the members of Mr. Forsyth's expedition. 



M. Potanin, as reported by the Sibir, reached Omsk 

 on June 27. The object of his expedition is the geogra- 

 phical, ethnographical, and economical exploration of 

 North-western Mongolia, for which purpose 9,400 roubles 

 were allowed by the Geographical Society and by the 

 Government. He will be accompanied by his wife, M. 

 Posdndeff, linguist, M. Raphailoff, topographer, M. Bere- 

 soffsky, volunteer, and M. Kolomiitseff, zoologist, sent 

 by M. Severtsoff. Starting from the Zaisansky post on 

 the Irtish, M. Potanin will follow the steppe-valley of 

 the Black Irtish and proceed to Urumga, Khobdo, the 

 Oobsa-nor. For winter-quarters he will then go south, 

 through Oolassootai to the eastern parts of the Tian 

 Shan. During the following summer, taking a northern 

 course, the expedition proposes to reach the sources of 

 the Jenissei and the Kossogol lake, returning south again 

 for the winter to the eastern foot of the Shangai-alin and 

 to the expansion of the Onguiin river. During his stay 

 in Omsk, M. Posdndeff has assiduously visited the town's 

 archives, and has found some very interesting documents ; 

 for instance, letters from the Telengoot chiefs written in 

 Kalmuck with Mongolian alphabet, whilst now the Telen-r 

 goots do not use any written language. 



