October 5, 1895] 



NATURE 



549 



tude. The first really effective survey was ihat of Messrs. 

 Lewis and Clarke in 1S04-1806. In 1815 Land Office 

 surveys were commenced. After the admission of the State 

 to the Union in 1820 more .^ccurate surveys were required 

 to fix the boundary lines, but these had to be rectified in 1850, 

 when serious discrepancies were found. Really trustworthy 

 surveying was only begun when the Coast and Geodetic Survey 

 commenced a triangulated line across the state in 1871. The 

 Mississippi and Missouri River Commissioners subsequently 

 rectified the mapping of the rivers, and now the topographical 

 survey of the State is being carried out by the U.S. Geological 

 -Survey, which has executed maps of one-third of the area 

 on the scale of about two miles to the inch. The Missouri 

 Geological Survey also makes a topographical map of selected 

 parts of the State on the scale of about one mile to the inch. 



THE OBSERVATORY ON MOUNT BLANC. 



A S briefly announced in our Notes last week, Dr. Janssen has 



recently visited the observatory on Mount Blanc. In the 



current Comptes Rcndiis he gives an account ot the expedition 



from a scientific point of view, and the following is a translation 



of his description : — 



We left Chamonix on September 8, at 7 a.m., and arrived 

 at the summit on September 11, at 2.30 p.m. The observatory 

 was then in front of us. This construction has several floors, 

 of which the framework, formed by large and massive beams 

 crossed in all directions in order to ensure the rigidity of the 

 whole, produces a deep impression upon the mind. One won- 

 ders how it has been possible to transport the edifice to this 

 altitude and fix it on the snow. However, if the conditions 

 offered by the hard, permanent, and little mobile snows of the 

 summit are carefully considered, it is soon recognised that the 

 snows are able to support very considerable weights,^ and that 

 they will be only slightly amenable to displacements, which 

 will render it necessary to straighten again the construction which 

 has been fixed upon them. 



On my arrival I made a rapid survey, and saw that the 

 construction had not been sunk in the snow as much as I had 

 stipulated of the contractors. I do not approve of this. My 

 guides and myself then took possession of the largest under- 

 ground room. I intended at first to fix the instruments for 

 enabling observations to be commenced immediately, and the 

 provisions were left on the Rocher-Rouge. This circumstance 

 put us in a state of perplexity, for the weather suddenly became 

 very bad, and we had to remain two days separated from the 

 stores. The storm lasted from Tuesday until Tliursday morning. 

 Beautiful weather then set in, and I was able to begin the 

 observations. 



The observations have for their principal object the question 

 of the presence of oxygen in the solar atmosphere. The Academy 

 knows that I worked at this important point during my ascen- 

 sions to the Grands-Mulets (3050 metres) in 1888, and at 

 M. Vallot's observatory in 1890. 



But the novelty of the observations of 1893 lies in the fact that 

 they have been effected on the very summit of Mount Blanc, and 

 that the instrument employed is infinitely superior to that of 

 the two preceding ascensions. At the first, in fact, a Duboscq 

 spectroscope incapable of separating the B group into distinct 

 lines was employed, while the instrument about to be employed 

 at the summit of Mount Blanc is a grating spectroscope (the 

 dispersive piece of which I owe to the kindness of Rowland), 

 with telescopes having a focal length of 075 and showing all 

 the det.iils of the B group. This circumstance is of considerable 

 importance, for it may lead to the discovery, in the constitution 

 of the group in question, of valuable elements for measuring in 

 some way the effects of the diminution of the action of our 

 atmosphere as one ascends into it, and, 'accordingly, to determine 

 whether this diminution corresponds to total extinction at its 

 limits. In fact we shall learn whether or no the double lines 

 which make "p the B group diminish in intensity as their re- 

 frangibiliiies diminish ; that is, as their wave-lengths increase. 



This circumstance may perhaps be employed with profit, if 

 not to measure, at least to observe the diminution of the action 

 of the selective absorption of our atmosphere. It has been 

 ascertained that the most feeble doubles fade away one after 

 the other as the atmosphere is ascended, that is to say, as the 



* ' -See Comptis Kcmfits f t an account of experiments made at Mention on 

 the resistance of slightly comf ressed snow. 



barometric pressures 



0"566 1 and multiply it by thir- 



absorbing action is diminished. Thus, under ordin.ary 

 circumstances, at the surface of our seas or upon our plains, 

 thirteen or fourteen doubles can be seen, not reckoning that 

 which is known as the head of B. 



But even at Chamonix, that is at an altitude of 1050 metres, 

 the thirteenth double is very difficult to make out, and at the 

 Grands Mulets (3050 m.), it is only possible to see from the tenth 

 to the twelfth, while at the summit of Mount Blanc I cou'd 

 hardly go beyond the eighth. 



It is not to be supposed that we establish a proportionality 

 between the numerical diminution of the doubles and that of 

 the atmospheric action. The law is evidently of a much more 

 complex character. But this diminution, especially when con- 

 sidered in connection with the experiments made with tubes 

 full of oxygen, and able to reproduce the series of atmospheric 

 phenomena to which we have referred, is sufficient for us to con- 

 clude that the B group would totally disappear at the limits of 

 our atmosphere. It is remarkable, however, that if we take 

 the co-efficient 0566 that represents the diminution of atmo- 

 spheric action at the summit of Mount Blanc according to 

 /o"43 

 Vo-76' 



teen — the number that represents the doubles clearly visible on 

 the plain — we obtain T.\ as the result ; that is to say, very 

 nearly the number (8) doubles that can be seen by me on the 

 summit of Mount Blanc. 



This result is certainly remarkable, but I repeat that, in 

 my opinion, it is only by the comparison with tubes reproducing 

 the same optical conditions as nearly as possible, that any defi- 

 nite conclusions will be obtained. These comparative experi- 

 ments have already been commenced in the laboratory of 

 Meudon Observatory, and they lead to the same result, viz., 

 the disappearance of the groups A, B, and o at the limits of the 

 atmosphere. On account of the importance of the question, 

 however, the experiments will be repeated and completed. 



The question arises as to whether the high temperatures to 

 which solar gases and vapours are subjected are not capable 

 of modifying the power of selective absorption, and particularly 

 whether the absorption of oxygen which takes place in the 

 sun's atmosphere would not be altogether different from that 

 indicated by the experiments which have been made at ordinary 

 temperatures. 



I have already instituted experiments with the idea of reply- 

 ing to this objection. 1 shall give an account of them to the 

 Academy indue course, but I may say that the absorption spec- 

 trum of oxygen, either the line spectrum or the unresolvable 

 bands, do not appear to be modified in an appreciable manner 

 when the oxygen is raised to temperatures of about 400 or 500 

 degrees. 



On the whole, I think that observations made on the summit 

 of Mount Blanc give a new and much sounder foundation 

 to the study of the question of the purely telluric origin of the 

 oxygen groups in the solar spectrum, and lead to the conclu- 

 sions previously stated. 



Independently of these observations I have also given some 

 attention to the transparency of the atmosphere of this almost 

 unique station, and to the atmospheric phenomena which are 

 included in such an extensive view, and across such a great 

 thickness. I shall speak of this on a future occasion. 



The observatory, of course, is not completed. There yet re- 

 mains much to be done independently of interior arrangements 

 and the installation of the instruments ; but the great difficulty 

 has been overcome, for we are free to work, and no longer have 

 to reckon with the snowstorms ; the rest will follow in due 

 course. 



I hope that the observatory will soon be able to offer a much 

 more comfortable sojourn than I have had there ; but that will 

 depend upon the weather. Be this as it maj-, I regret nothing. 

 I strongly wished to see our work in position, and still more 

 fervently desired to inaugurate it by observations which are ever 

 in my mind. I am fortunate at having been able to realise my 

 desires in spite of some dfficulties. 



IRON AND STEEL INSTITUTE. 



A VERY successful meeting of the Iron and Steel Institute 

 ■'*• has just been held in Darlington, commencing on Tues- 

 day, September 26. The President, Mr. E. Windsor Richards, 

 occupied the choir There was a very good list of papers, 



NO T249, VOL. 48] 



