320 



NA rURE 



[August 2, 1894 



promised are : K. von Zittel, Palaeontology ; M. de 

 Bertrand, Structure of the Western Alps ; A. Heim, 

 Geology of the Environs of Zurich ; A. Michel-Lt5vy, the 

 Unification of Petrographical Nomenclature ; E. Suess, 

 Tectonic Geology. There will also be papers on glacial 

 geology ; and Captain Marshall Hall will submit a pro- 

 posal for an international survey and record of glaciers. 



At the Zurich meeting, however, papers and discussions 

 will form but a comparatively small part of the work. 

 Excursions have always played a prominent part in the 

 arrangements for the various meetings ; but hitherto they 

 have been mainly made after the close of the Congress. 

 At Zurich the Congress will practically divide into five 

 excursion sections, starting on September 3, traversing 

 the .Alps in different directions, and all converging on 

 Lugano, where the closing meeting will be held on 

 September 14. These excursions would alone make the 

 Zurich meeting memorable ; they have been planned to 

 include the most interesting districts of the Swiss .Alps, 

 and to facilitate the study of many intricate problems 

 concerning the structure of the mountains and the petro- 

 graphical nature of the rocks. After investigation of the 

 northern flanking ranges of folded secondary rocks, the 

 central crystalline zone will be crossed, and in some cases 

 glacial phenomena can be well studied. Prof. Heim will 

 conduct a party over the country which he has so well 

 described, starting from St. Gall and crossing the Alps of 

 Glarus, the Vorder Rhein, and the eastern Lepontine 

 Alps ; Prof. Schmidt will conduct the party from Schwtz 

 over the St. Gotthard ; Prof. Baltzer, starting at Lucerne, 

 will take a line some miles further west ; Prof. Schardt, 

 starting at Bulle, will traverse the western end of the 

 Bernese Alps and part of the Pennine Alps, and will reach 

 Lugano by the Simplon. These four excursions are for 

 pedestrians only, and those only are invited who are 

 accustomed to long walks and climbing, hard beds, and 

 frugal living. A more elaborate circular tour in the Alps 

 will be conducted by .M.M. Ruffieux and Ruchonnet, of 

 Lausaunne ; this will traverse a wider district, and the 

 work will be done with less fatigue. Profs. Renevierand 

 GoUiez will be the scientific directors of this tour. 



Supplementary excursions will start from Lugano 

 after September 14, one of which, conducted by Profs. 

 Bruckner, Du Pasquier, and Penck, will study the glacial 

 phenomena of the Italian Lakes, thence by the Tyrol 

 to .Munich, and finally to the Lake of Constance. 



Before the Congress there will be excursions in the 

 Jura — five for pedestrians— as follows: Krench Jura, 

 M. Schardt ; Vaudois Jura, M. Jaccard ; Bernese Jura, 

 M. Rollier ; Bale and the Argovian Jura, M. C. Schmidt ; 

 Argovian Jura and Soleure, &c., NL Alurlberg. There 

 will alio be a long circular lour in the Jura by MM. 

 Ruffieux and Ruchonnet, with MM. Rcnevier and 

 Gollitz as scientific directors ; the latter part of this 

 will be much devoted to glacial questions, and will 

 therefore be preparatory to the special glacial excursion 

 starting from Lugano. 



Arrangements have been made for inclusive charges 

 for all these excursions. For the pedestrian tours, they 

 are 50 or 60 francs for the Jura excursion of five or 

 six diays each, and 300 francs for the circular Jura tour of 

 fourteen days. 



For the longer excursions in the Alps, after the Con- 

 gress, the prices are from 150 to 250 francs for the 

 pedestrian tours of eight to thirteen days, and 400 francs 

 for the circular tour of thirteen days. 



A guide-book to the various excursions is in prepara- 

 tion. This will contain about 300 pages of text, and will 

 be amply illustrated by plates and sections ; it will form 

 a most useful handbook to the geology of .Switzerland. 

 A new geological map will also be |jublishcd,oii the scale 

 of I : 500,000; this will be a reduction of the official 

 maps of the Swiss Geological Survey, which is now 

 completed. 



NO. 1292, VOL. 50] 



Special guide-books to the important geological col- 

 lections at Lausanne and Zurich are in preparation. As 

 usual at such meetings, geological maps and other pub- 

 lications, photographs, specimens, &c., will be exhibited. 



W. TOPLEY. 



THE DISCS OF JUPITER'S SA TELLITES. 



THE discussion which is now taking place between 

 two well-known observers — namely, Profs. Pickering 

 and Barnard — as to the forms which the satellites of the 

 planet lupiter assume at various times, is one not only of 

 absorbing interest, but, moreover, of a nature somewhat 

 delicate, for the bodies in c[uestion are so minute as 

 to baffle any but the very best and trustworthy observers. 

 Such observations, then, to be of any value at all, must 

 be either made in the clearest of atmospheres with a 

 moderately large aperture, or in a moderately clear 

 atmosphere with a very large aperture. Considering these 

 two conditions, one would doubtless think that the larger 

 the instrument the more chance there would be of finding 

 out the shape of a body, and with a very clear atmo- 

 sphere in addition these chances would be very greatly 

 increased. On the other hand, however, we have the 

 facts still in our mind of Schiaparelli's wonderful observ- 

 ing powers, which enabled him to notice the doubling of 

 the canals of Mars with his small aperture long before 

 they were declared "double" by other observers. In 

 this case one would have thought that such an observa- 

 tion would have been more easily observed with large 

 apertures than with the small telescope which was at his 

 disposal. 



Let us, however, turn to the facts at hand with regard 

 to the satellites that are now under discussion ; but first 

 a few words with regard to the instrumental equipment 

 employed and the observing stations. 



Prof. Pickering's observations have been made at the 

 Observatory that is situated near Arequipa, in Peru, at 

 an altitude of more than eight thousand feet, where 

 the sky during a large part of the year is ne.irly cloud- 

 less. The telescope employed has shown that there is a 

 remarkable degree of steadiness in the atmosphere, and 

 night after night atmospheric conditions jirevail, which, 

 as he says, " occur only at rare intervals, if ever, in 

 Cambridge." Several of the diffraction rings surround- 

 ing the brighter stars are visible, close doubles in which 

 the components are much less than a second apart are 

 readily separated, and powers can be constantly em- 

 ployed which are so high as to be almost useless in Cam- 

 bridge. In fact, he says that in many researches the 

 gain is as great as if the aperture were doubled. The 

 aperture of the refractor employed is i 3 inches. 



Prof. Barnard has made his observations, on the other 

 hand, with the now well-known 36-inch refractor of the 

 Mount Hamilton Observatory, a description of which 

 here would be unnecessary ; suflice it to be mentioned that 

 Prof. Burnham has increased the number of double stars 

 by about 200 during his brief use of this instrument, 

 most of which are beyond the reach of the majority of 

 telescopes. 



Turning now to the observations of the satellites 

 themselves, we find the first account of Prof Pickering's 

 observations in the March number of Astronomy and 

 Astro- I'hysics for the year 1892. 



On October 8, a series of measurements was made of 

 the diameters of the satellites. On the next evening it 

 was noticed that the disc of the first was not circular but 

 very elliptical. Early observations on the tenth con- 

 firmed the measurements made on the eighth, but after 

 an examination of the other satellites the first was again 

 measured, when, as Prof. Pickering says, "to my 

 astonishment, instead of showing an elliptical disc, it 

 showed one that was perfectly circular, precisely like the 



