Oct. 23, 1879] 



NATURE 



605 



French engineers are investigating the territory from the 

 starting-point of the future line to the Laghouat in the 

 south. From that point as far as the line from Ain Sala 

 au Khat military-technical expeditions are to prepare the 

 way. Besides this the Oran Geographical Society and 

 the Vice-Prefect of Tlemcen are to send caravans along 

 the frontier of Marocco. MM. Soleillet and Duveyrier 

 will travel on their own account for the same object. 



A MEETING took place at Dortmund on the 4th inst., 

 which had for its object the discussion of a project for 

 connecting the Rhine and the Weser by means of a canal. 

 The Government presidents of Westphalia, of the Rhine- 

 land, and of Hanover were present. The canal will be 

 constructed vi& Ruhrort and Heinrichenburg, but it is 

 undecided as yet whether from the latter place it will 

 proceed to Emden or to Minden. Special committees 

 were formed for the purpose of further investigating the 

 latter question. 



Messrs. Schmidt and Gunther, of Leipzig, will 

 shortly publish an elaborate work on India by Emil 

 Schlagintweit. The work will appear in thirty-five parts, 

 and will be profusely illustrated by eminent artists. 



A parliamentary caravan comprising about twenty 

 members of the French Senate and Chamber of Deputies 

 is travelling all over Algeria in order that the legislators 

 may become acquainted with the peculiarities of the land 

 and inhabitants. The tour will be terminated at the end 

 of the month. 



THE PLANETS OF THE SEASON 



SELDOM could the aspect of the nocturnal sky be 

 more attractive to the student of planetary pheno- 

 mena ; seldom has his inquiring gaze been repelled more 

 pertinaciously by 



"Vapours and clouds and storms," 



than during the past anomalous season ; and our English 

 climate has more than maintained its accustomed forbid- 

 ding character, just when a few nights of transparency 

 would have been especially welcome. Better things, 

 however, may still be in store ; and in that hope the 

 following remarks on the distinctive features of the 

 present ornaments of our midnight sky may be admis- 

 sible as possibly of some suggestive value to compara- 

 tively inexperienced observers. 



We have for some time past had a simultaneous pre- 

 sentation, under the favourable circumstances of proximity 

 to the earth and a greatly improved altitude as compared 

 with recent oppositions, of three peculiarly remarkable 

 objects, the most magnificent, the most ornate, and — so 

 to speak, the most earth-like member of the planetary 

 family. Each holds his own pre-eminence on his own 

 ground ; each bespeaks especial study from his own indi- 

 vidual character; and it is probable that some acquaint- 

 ance with foregone results may economise time and labour 

 by enabling us to leave on one side what is already, in 

 comparison, sufficiently known. 



The examination of Jupiter is not at present quite as 

 satisfactory as it would be with as much north as he has 

 south declination ; it is always, however, greatly facili- 

 tated by the broad expanse of his noble disk, and a bril- 

 liancy so great as to have occasioned a suspicion of 

 unborrowed light, emanating from internal incandescence. 

 While, however, the shadows of the sateUites upon his 

 surface arc so intensely black, and the satellites them- 

 selves so utterly invisible in eclipse, it is evident that any 

 accession of luminosity from such a cause, even if it 

 exists, must be quite insensible in the general efiect, 

 which can only be ascribed to an extraordinary reflective 

 power in the whiter portions of the globe. A diminution 

 of brightness towards the limb, which might be antici. 



pated on optical grounds, and is frequently demonstrated 

 by the reversal of the aspect of a passing satellite from 

 light to dark, or the contrary, is nevertheless not distin- 

 guishable by the eye, nor even when a portion of the 

 light has been intercepted by darkening glasses ; it is, 

 however, apparent if the screen is deep enough in tint to 

 extinguish the satellites. This was scarcely to be ex- 

 pected. It might have been supposed that a central 

 region so bright as to exhibit a white disk in front of it 

 as dark, even to blackness, by contrast, could not escape 

 being itself strongly contrasted with a border so much 

 fainter that the same disk appears luminous on it as a back- 

 ground. Yet the difference is not obvious ; and no other 

 cause can be assigned excepting the imperceptible grada- 

 tion. The idea of absorption in the upper region of an 

 extensive atmosphere, not otherwise manifesting its 

 existence, has indeed been entertained ; but it seems 

 unnecessary. The observed decrease of brilliancy is only 

 what would result from obliquity of incidence in the solar 

 rays, and it probably exists in no greater proportion than 

 is due to that cause. The atmospheric hypothesis might 

 indeed have been directly tested, by comparing the 

 brightness of the satellites when near the limb in the 

 superior and inferior portions of their orbits, had their 

 light been sufficiently equable ; but its variations are too 

 evident and at the same time too irregular to render such 

 comparisons satisfactory. A long course of observation 

 might indeed eliminate these discrepancies, but it is 

 questionable whether the result would repay the labour. 

 Nevertheless the fact referred to, of the incompetence of, 

 at least, ordinary vision to detect the diminution of light 

 towards the limb is worth attention, as leading to infer- 

 ences rather unfavourable to the sensitiveness of the eye 

 in some of the processes of photometry. At a consider- 

 able distance from opposition, when the terminator 

 encroaches slightly on the elliptical form of the limb, the 

 defalcation of light on that side may be readily detected. 



As to the real nature of that magnificent globe we are 

 compelled to admit an embarrassing amount of ignorance. 

 We see indeed that it is encompassed by an envelope, 

 subject to occasional disturbances of a nature which on 

 our earth would necessarily infer the extensive prevalence 

 of vapour, sometimes in tranquil suspension, at others 

 either agitated by rapid currents, or subject to equally 

 speedy processes of precipitation and solution. Beyond 

 this we can hardly be said to know anything. Jupiter is 

 in no respect an enlarged resemblance of the earth. With 

 so little similarity in point of density and gravitation — 

 with so slight a diversity of seasons — with such rapid 

 interchange of day and night — could we be transported 

 there, we might probably find ourselves as among the 

 imagery of an incomprehensible dream. Vapour we 

 might recognise — and vapour occasionally in a state of 

 rapid change ; but possibly not the vapour of water ; and 

 whether exhibiting itself in the luminous or shady spaces 

 could hardly be decided by a mere comparison with 

 terrestrial clouds. These would no doubt be to a distant 

 eye brighter than the surface beneath them, but among 

 so much that is dissimilar a single point of analogy would 

 hardly bear much weight ; it is, however, the more 

 probable alternative that the dark bands are the trans- 

 parent part of the atmospheric envelope, from the fact 

 that these become less distinct towards the limbs. The 

 obliteration is not indeed always apparent, and is often 

 absolutely imperceptible in a sharply defining instrument ; 

 but it has been frequently referred to, and if these ideas 

 are correct, it may probably be found that in proportion 

 to the darkness of the belts will be the nearness of their 

 approach to the edge of the disk. The disappearance of 

 dark spots near the limb would be accounted for by the 

 rules of foreshortening in perspective. 



The tendency to an equatorial arrangement in these 

 streaks is one of the most familiar features of the planet ; 

 and almost self-evidently connects itself with the astonish- 



