382 



NATURE 



\SepL 5, 1872 



the possible variations of visibility of the numerous spots and 

 cra'erlets upon the floor of Plato under the same conditions of 

 illumination. That now presented is directed chiefly to the dis- 

 cussion of the various streaks and bright patches which interlace 

 the spots and craterlets. One interesting and important change 

 has been fairly shown ; the Moor of Plato becomes darker with 

 the increase of the sun's altitude, Mr. Birt has offered an explana- 

 tion of the phenomenon ; whatever be the true cause of thi-i 

 change, it is very difficult to account for it by the ordinary laws 

 of reflecti-xi ; when vv-e consider the varying aspect of the 

 streaks at the same time of the luni-solar day, we cannot but 

 think that, ^^■ith careful observations made with powerful instru- 

 ments such as the Newall refractor and many others, we may 

 be able to confirm or otherwise a physical explanation of these 

 curious changes involving the existence of certain gases and 

 vapours upon the surface of the moon. Tlie Committee can 

 only look upon the study of Lunar physics as in its infancy, and 

 they trust that in future years the Association will not overljok 

 tliis important branch of astronomical inquiry. 



After reading this Report of the Committee, Mr. Birt read 

 extracts from his report of the discussion of the streaks on the 

 floor of the lunar crater Plato which had been entrusted to him, 

 the opening portion was to the following effect : — 



" In completing the task assigned to me of discussing the 

 observations of the streaks on the floor of Plato, I have been 

 desirous of including every, even the most minute circumstance 

 bearing on the exhibition of phenomena that may possibly illus- 

 trate the condition of a small portion of the moon's surface from 

 April 1S69, to April 1S71. Drawing my conclusions from the 

 experience of twelve years, I feel that I miy confidently say that 

 it may be some years before another series of observations of a 

 particular region will be undertaken with the view of so closely 

 examining the spots and streaks characterising it, unless a staff 

 of efficient obser\:ers be organised, with the provision of a fund 

 sufficiently ample to defray all the necessary expenses. The 

 work is a difficult one, the staff should consist of not less than 

 six devoted observers who would independently and most pro- 

 bably, as in the present case, work with instruments of varying 

 ajierture, an 1 carefully record all their observations. Tlie 

 princip.al qualification is a keen eye for the appreciation of deli- 

 cate variation! o^ tint and the detection of minute spots of lig'it, 

 with a readiness of referring the.n by estimation and alignment 

 to the respective localities of the region on which they are seen. 

 The observations shiuld not be allowed to accumulate, but 

 should be forwarded at once to an experienced selenogi-aphcr 

 charged with the work of arranging and discussing them. 

 Taking into consideration the results of the discussions of the 

 present and previous years embodied in the two reports, it 

 appeal's that in order to confirm these results and to open up 

 new inve ligations the requisite time cannot well be fixed at less 

 than three years — five would most probably afford the best 

 results. 



" The results of the present work may be briefly characterised 

 as confirming by a direct reference to the sun's altitude above 

 the horizon of Plato t'ne supposition that variations of tint in 

 some measure depend on increasing and decreasing altitudes. 

 The ascending and descending branches of the curve obtained 

 from independent estimations of tint by the several observers, are 

 sufficiently near those of the sun's altitude to enable me to 

 delineate a normal curve representative of the sun's influence in 

 darkening the floor of Plato, or else in overspreading it with 

 something of the nature of a dark covering as liis rays strike the 

 surface at the increased angle of about forty degrees. While this 

 darkening influence comes out most unmistakeably, there are 

 variations in the lighter and darker portions of the floor which 

 ae quite irreconcileable with solar influence of a grafual 

 character. The treatment of the observations uuiler intervals of 

 the luni-solar day fails to bring out any reguhrity in these varia- 

 tions, and it is only by treating the observations cltromlogkally 

 that the true sequence of the changes can be detected." 



Mr. Birt proceeded to notice that in order to assist in showing 

 more distinctly the changes observed on the floor of Plato and 

 their connection with certain supposed agencies, he hid intro- 

 duced the hypolhesis of a dark obscuring medium overlying the 

 surfaces of the lover parts of the moon ; he did not insist upon 

 this hypothesis further than its utility in connecting the observa- 

 tion;. The Report, which was rather voluminous, referred 

 especially to the influence of the sun on the floor of Pla'o ; an 

 examination of the changes recorded in August 1S69, formed a 

 separate portion of the Report as well as the history of a single 



streak from its first detection in September 1869, to the c'ose of 

 the observations. A considerable portion of the Report con- 

 si!led of " Notes " furnished by the several observers. 



SECTION B— Chemical Science 

 On Filiform Native Stiver, by J. II. Gladstone, F.R.S. 

 The object of this communica'ion was to show that metaliic 

 silver might be obtained artificially in the sa-ne filiform condition 

 in which it fre([uently occurs in a mineral, and thus to throw 

 light on the origin of this native variety. Specimens of the 

 metal were exhibited, from Kongsberg in Norsray, associated 

 with calc-spar, and from Chili, associated with greenstone, and 

 in each case the silver resembled twisted threads or wire; non- 

 crystalline, but often bending at sharp angles. Under the micro- 

 scope were exhibitel precisely similar threads of silver produced 

 by tlie decomposition of nitrate of silver by sub oxide of copper. 

 The latter substance is partly dissolved and partly converted into 

 the black oxide, while filaments of the white metal shoot forth 

 and bend in every direction. Most of these are extremely fine, 

 pe.-haps jTSTTini of an inch in thickness, so that, as was said, a 

 gramaie of such wire would stretch from London to Brighton. 

 Since sub-oxide of copper is no rare metal, it seems probable 

 that filiform native silver may often, if not always, originate 

 from it. 



SECTION C— Geology 



On the Occurrence of a remarkable Grottp of Graptolites in the 

 Arenig Rocks of SI. David's, South Wales, by John Hopkinson, 

 F.G.S., F.R.M.S. 



In a series of black, iron-stained shales, about 1,000 feet in 

 thickness, which form the lowest beds of the Silurian rocks in 

 the immediate vicinity of St. David's, the author noticed the 

 occurrence of about twenty species of graptolite, which, he coa- 

 siilered, famished conclusive evidence of the equivalency of these 

 beds with the Quebec group of Canada, the Skiddaw slates of 

 Cumberland, and the Arenig rocks of Shelve. 



The Graptolites, of which there are more than twenty species, 

 were collected in the lower beds of the series at Ramsay Island 

 and Whitesand Bay. Of the true Graptolites, or Klialidnphora, 

 the only genera of undoubted occurrence are Didymo:^) aptus, 

 Tetragraptus, and P/iyllograptus. Didyniograptus is represented 

 by five species, three of which — D. extensns WiXi, D. patuhis. 

 Hall, and D. pcnnalulus Hall — are characteristic of the Quebec 

 group and the Skiddaw slates, D. patulus also occurring in the 

 Arenig rocks at Slielve ; the other species are new. Of Tetragrap- 

 tus but one species, T. serrus Brong., aho a Quebec and Skiddaw 

 form, has been found. Phyllograptus also is only represented by 

 a single species, which is new. There is also another new 

 species — a very peculiar branching form referred provisionally to 

 Loganogiaptus. The absence of any specimens undoubtedly 

 referable to Dictyograptiis is remarkable, as this is a common 

 Quebec genus. Diplograptus and Climacograptus, genera of 

 very rare occurrence in the Quebec gi'oup, have not as yet been 

 found here. 



Of the allied forms, all the genera of the so called "Dendroid" 

 Graptolites, so ch.rracteristic of the Quebec group, are present in 

 the St. David's beds. Phyllograptus is represented by two new 

 species, and Dendrograptus by five species, three of wliich — 

 D. divergens Hall, D. flexuosus Hall, and D. striitus Hall — are 

 at present only known to occur elsewhere in the Quebec group, 

 the o'her not being new. Callograptus is also represented by 

 five species, three — C". elegans Hall, C. diffusus Hall, and 

 C. Salteri Hall — being Quebec forms, and two being new ; and, 

 lastly, o{ Dictyonema but one species, which is new, has been 

 found. Many obscure impressions referred to the genus Retiolites 

 aUo occur, one species seeming to agree perfectly, as far as its 

 state of preservation allows of comparison with Prof. Hall's 

 figures, with his A", ensiformis of the Quebec group. Another 

 appears to be distinct from any species yet figured. 



The Graptolites and their allies are now thus known to be 

 represented in the Arenig rocks of St. David's by nine genera 

 and about twenty-two species. Of the true Graptolites three 

 genera — namely, Tetragraptus, Loganograptns, and Phyllograptus, 

 are ex dusively confined to the horizon of the Quebec and Skid- 

 daw groups. Tlie remaining genus, Didyniograptus, is repri- 

 sented in higher rocks but by one species, D. Murchisoni. \Viili 

 this exception, Didyinop'aptus is exclusively an Aren'g ge.ius 

 occurring in rocks of this age in Canada, Cumberland and Shrop- 



