September 15, 1892] 



NATURE 



483 



extraordinary phenomena to which I alluded in my last letter 

 may account for this. 



Besides, I have gained nothing by waiting, and at the present 

 time, after successive delays which I much regret, I am hardly 

 further than I was a month ago. Owing, perhaps, to the 

 images being Ie-;s satisfactory, or to the phenomena in question 

 not having recurred, nothing has been added to my first 

 observations. 



The phenomena alluded to are brilliant projections, com- 

 parable in colour and brightness to the southern pole cap, 

 observed on three different o:casions — viz., June lo and July 2 

 and 3, on the western lioab of the planet. 



The last time, July 3, I was able to observe the several phases 

 of this singular appearance. On that day the luminous point 

 began to einer^^e on the edge of the disc at I4h. iiui. (local 

 astronomical time), very faint at first ; then I saw it gradually 

 increa-e, pass through a maximum, and then diminish, to dis- 

 appear finally about I5h. 6in. The facts would not have been 

 different had it been a case of an elevation of the surface of 

 Mars traversing the illuminated edge of the disc by the simple 

 effect of the rotation of the planet. The phase which affected 

 the western limb of the planet at that time, could only modify it 

 in amount and in duration. The previous night, July 2, I had 

 seen the crescent in a phase approaching the maximum, at 

 I4h. lom., and 1 was able to follow the bright point up to its 

 complete disappearance at I4h. 40m. 



On July 2 and 3 the things happened in the same part of the 

 disc, about the 50th parallel of latitude, and with a retardation 

 of half an hour against the previous day, as usual for a thing 

 taking place in the same region of the planet. 



The first observation of this kind goes back as far as June 10, 

 when it lasted from I5h. 12m. to about l6h. 17m. This time 

 the bright point occurred in the vicinity of the 30th southern 

 parallel, probably in the southern portion of the isthmus Hesperia 

 of Schiaparelli's chart. 



I may add that during these observations the portion of the 

 disc adjoining the small protuberance has always appeared to me 

 slightly deformed and as if raised. 



Such are the facts. I shall not attempt to interpret them. 

 They presented themselves with such clearnes-; that it is hardly 

 possible to consider them as the result of any illusion. 



On the other hand, since it is a question of projection beyond 

 the disc of at least one or two tenths of a .second of arc, that is 

 to say,- of phenomena -at- a- height of more than 30 or 60 km., 

 one feels overwhelmed by such numbers, to which we are not 

 accustomed on our globe, and it is undoubtedly luminous phe- 

 nomena only which could explain heights like that. 



The southern snow cap has been the object of several 

 measurements, which will be published with the drawings of 

 this opposition. This Cap has notably diminished in the last 

 two months ;' it \i. In fact, shifting ; it is traversed by at least 

 two black lines, a kind of crevices analogous to those which I 

 announced in 1888 in the case of the northern cap. The first 

 of these lines was seen at the end of June, the second on 

 August 8. 



The outline is now more irregular than in the past ; in par- 

 ticular there is seen, between the meridians of 300° and 0° 

 (Schiaparelli's map), a deep black hollow which grows steadily. 



Although the actual conditions are not very favourable for the 

 canals (at least for a portion of them), several are well visible ; 

 some are apparent enough to convince the most prejudiced 

 observers. 



Two of our drawings of the Great Syrtis, made at widely 

 different dates, indicate some slight changes in the most north- 

 erly portion of this sea. They are no doubt due to mists and 

 clouds, which have sometimes appeared to me to invade the 

 northern regions on the east of this Great Syrtis, hiding the 

 canals which traverse them, and only allowing us to see their 

 most southern portion. 



Our drawings of the Lake of the Sun, when compared with 

 those of M. Schiaparelli, also indicate some changes of detail in 

 the aspect of the lake itself and of the seas and canals surround- 

 ing it. 



The most interesting observation of this month is the 'one I 

 have made, on August 6, of a very bright point placed precisely 

 a little to the north of this Lake of the Sun. This point, which 

 struck me by its extraordinary radiance, could not be seen the 

 next day ; if it still existed — the images were not so good as the 

 previous night — it was certainly much less luminous. 



This phenomenon, and the analogous phenomena sometimes 



NO. I 194, VOL. 46] 



noticed on the surface of the planet, are perhaps not without 

 some relation with the appearances of the limb which I have an- 

 nounced. Future observations will no doubt inform us on this 

 subject. 



I should perhaps have still deferred sending this letter if I 

 had not, within the last few days, received from Mr. Newcomb 

 the extract of a journal, in which it is reported that the Lick 

 astronomers have also observed the luminous projections on the 

 edge of the disc. 



I may add that in the beginning of July I had imparted my 

 observations to M. Andre, director of the Lyons Observatory, 

 who happened to be on a visit to Mont Gros, and whom I had 

 invited to come on the 5th and verify the strange appearances 

 which I h^d told him of. Unfortunately, the sky remained 

 obscured all night, and my project could not be carried out.. 



SCIENTIFIC SERIALS. 



The American Mn/eoro/o^-ical lotemal ior August conU\n<i :— 

 Synoptical sketch of the progress of Meteorology in the United 

 States, by W. A. Glassford, and reprinted from the annual 

 report of the chief signal officer for 189 1. From this summary 

 it appears that Isaac Greenwood, a professor of mathematics in 

 Harvard College, prepared a form for observations at sea in 1728, 

 thus anticipating the efforts of Lieut. Maury by more than a 

 century. Observations of temperature and rainfall were begun 

 in Charleston in 1738, and were soon followed by several other 

 series. In 1817, J. Meigs, Commissioner of the General Land 

 Office, proposed to Congress the establishment of meteorological 

 stations at each of the land offices, and as this proposal was not 

 adopted, he started a voluntary system among his subordinates, 

 and supplied registers for the purpose. This system lasted until 

 his death in 1822. The next service was established by the 

 Surgeon-General of the Army, in 1819, and was maintained, 

 with modifications, until 1854, when the records were handed 

 over to the Smithsonian Institution, and in due time were trans- 

 ferred to the Signal Service. The Patent Office, of which agri- 

 culture formed a division, and the Coast Survey also manifested 

 great interest in the science. The article contains a good review 

 of the labours of the principal American meteorologists. — Note 

 on winter thunderstorms ; by Prof. W. M. Davis. He asks 

 whether the convectional origin of thunderstorms in summer 

 implies a like origin for thunderstorms in winter, even though 

 they occur then at night, and he explains the reasons which 

 seem to favour this supposition. — Objections to Faye's theory of 

 cyclones ; by W. C. Moore. The writer attempts to show why 

 the generally accepted theories seem to him preferable to those 

 brought forward by M. Faye. The discussion is to be continued 

 in a future number. — Artificial rain ; by E. Powers. The 

 writer is the author of a work entitled " War and the Weather,'' 

 and he supports the view that rain can be artificially produced, 

 and endeavours to refute the objections ur^ed by Prof. W. M. 

 Davis and others. 



Wiedemann s Annalen der Physik itnd Chemie, No. 8. — On 

 the refraction of rays of great wave-length in rock-salt, sylvine, 

 and fluorspar, by H. Rubens and B. W. Snow. A series of 

 bolometric researches concerning the infra-red rays, to determine 

 the refractive indices of the three substances for light of various 

 wave-lengths up to A = 80,000. Fluorspar, though showing a 

 lesser dispersion than the other two in the visible portion, ex- 

 celled them enormously in the infra-red, hence it is specially 

 suited for the production of prismatic heat spectra. — Reflection 

 and transmission of light in certain jeolotropic structures, by 

 H. E. J. G. du Bois. An aeolotropic structure is a portion of 

 matter, generally plane, in which it is possible to fix upon 

 an optically favoured direction. This can be due to its 

 coarse macroscopic or its molecular and microscopic struc- 

 ture. In both cases vertically incident or reflected light will 

 be acted upon differently according as its plane of polariza- 

 tion is parallel or normal to the favoured direction. This 

 action is in general unequal as regards both the amplitude and 

 the phase of the two components. The objects experimented 

 upon were, in the first class, bright silver wire gratings, platinum 

 film gratings, scratched metal reflectors, and scratched glass 

 gratings ; in the second class, crystals of cobaltine and pyrites, 

 and a loaded steel mirror. In the case of the silver wire 

 gratings it was found that light polarized in a plane normal 

 to the direction of the wires was let through in greater intensity 



