254 



KNOWLEDGE 



[No\-EMBEB 1, 1890. 



conjunction with my impression that it does not always 

 appear precisely in the same place, I think that there may 

 possibly be two nearly equal stars there, too faint to pro- 

 duce distinct sensations on the retina. I see no other 

 stars whatever in the Nebula, nor have I the impression 

 that even the brightest parts would break up into stars, 

 even with far more optical power." 



As is well known, the Nebula has since been shown 

 to be gaseous ; but M. Trepied thinks that in the original 

 negative he has evidence of at least three small stars in 

 the brighter parts of the ring. It will be noticed that the 

 outline of the ring in M. Trepied's photograph is more 

 nearly circular than in Mr. Lassell's drawing. The de- 

 crease in brightness of the ring near to either end of the 

 longer axis of the elhpse is a marked feature of the 

 Nebula, as seen in the telescoise, and is shown in the 

 drawings of Tronvelot (Annals nf Hnnard Ohsemitori/, 

 Vol. 8, Plate 84) and Holden (WaMmitun Ohxen-dtions, 

 1874, Plate VI). The fainter light of the extremities of 

 the ellipse is also mentioned by Padre Secchi in the Descri- 

 ziiinc del Xitovo Dsnei raturio tld CoUeijio Eonuino, Eome, 

 1856, page 86. The decrease of brightness at the ends of 

 the ellipse is also very distmctly recognizable in M. Tre- 

 pied's photograph, and the southern side of the ring 

 appears distinctly brighter than the northern. 



The Nebula has also been photographed by Mr. Isaac 

 Roberts, 31st .July 1887, with an exposure of 20 minutes, 

 in the principal focus of his 20-inch reflector ; stiU more 

 of the Nebula is shown upon his photograph, and the ex- 

 posure has gone so far that the density at the ends of the 

 ellipse is barely distinguishable as less than that at the 

 sides. There is also a small projection or irregularity of 

 the ellipse at the south-west end, which is well shown in 

 the drawings of Trouvelot and Holden ; but, curiously, 

 neither of these observers show the star or nebulous mass 

 at the centre, which is so clearly shown in M. Trepied's 

 photograph as well as in that of Mr. Roberts, and in a 

 small photograph taken by Herr Von Gothard in 1886. A 

 valuable account of the chief drawings of this Nebula 

 made up to 1875, is given in a paper by Prof. E. IS. 

 Holden, published in the Monthlii Xatiax for December 

 1875. 



The following fuller list of drawings and descriptions of 

 the Nebula I owe to Mr. Sadler : — 



Herschel, .Sir John, " Phil. Trans.," 1833, Plate X., Mg. 

 29. Verv symmetrical ellipse. 



Earl of liosse, "Phil. Trans.,' 1844, Plate XIX., Fig. 

 29. A curious drawing, with jjarallel bands of nebulosity 

 in the interior, and wisps of nebulosity from the outside of 

 the ring. 



Sadler, " Engl. Mech.," Oct. 13, 1882, Misprint No. 2 for 

 No. 1. Symmetrical drawing, giving places of small stars. 



Brashear, " Engl. Mech.," Nov. 14, 1884. Symmetrical 

 ring, ellipse too narrow. 



U Arrest, " Instrumentum Magnum Equatoreum," Plate 

 11, Fig. 5. 



D'Anvst, " Siderum Nebulosorum Obs. Havnieuses." 

 Symmetrical sketch ; no central star. 



Holden, " Washington Observations," about 1876 or 

 1876. One of the best drawings, but no central star. 



Trouvelot, " Astron. Engravings from Harvard Obs.," 

 Vol. 8, Plate 34. Very good drawing ; several small stars 

 are shown in the nebulous ring, but none at the centre. 



Lamont, " Annall Munich Obs.," Vol. 17, Plate 11, Fig. 

 14. Small symmetrical drawing. 



Lassell. Drawing privately published ; copy in Astro- 

 nomical Society's rooms. 



(K M. Mitcliel, •' Sidereal Messenger, ' .July 1846. De- 

 scription, but no drawing. 



The photographs of .Jupiter and Saturn given in the 

 plate are remarkable as showing the very different actinic 

 action of the light from different parts of the discs. The 

 light from the Polar regions of both Jupiter and Saturn 

 has hardly impressed itself on the plate. The light from 

 the equatorial regions of Saturn also seems to be deficient 

 in actinic power. 



THE COAL-FIELDS OF CENTRAL FRANCE. 



By G. W. Rui.man, M.A., B.Sc. 



THE coal-fields of Central France present some in- 

 teresting peculiarities. In the north of that 

 country the coal-bearing strata are a continuation 

 of the Belgian coal-field, and, as was suggested 

 by Godwin-Austin in 1856, and practically proved 

 by the recent discovery of coal in Kent, are connected with 

 the South Wales and Somersetshire basins. 



The isolated, basin-shaped areas in which our coal 

 measures lie is attributed to folding, and subsequent 

 denudation of the higher and more exposed parts ; but 

 the coal of Central France occupies areas which were 

 depressions in the older rocks before the time of its for- 

 mation. 



The whole question of the origin of coal is a difficult 

 one, and admittedly unsettled. It is true there is a gene- 

 rally received theory, which assumes -that there was a 

 gradual sinking of the surface, with pauses during which 

 beds of vegetable matter were formed by terrestrial growths 

 in situ, but it confessedly presents many difficulties. 



One of the most important contributions to recent 

 geological literature is a work by M. H. Fayol on the 

 French coal-fields, in which the theory of growth in situ 

 is entirely rejected. At the commencement of his prac- 

 tical work in the coal-mines of Commentry, this gentle- 

 man, fresh from I'Ecole des Mines, and imbued with the 

 prevailing theory — la theorie iles tourlneries, he calls it — was 

 brought face to face with phenomena which, to him, were 

 inconsistent with it. Twenty-six years' experience, dming 

 which he has had exceptional opportunities for studjong 

 the question, has led him to discard the generally received 

 idea, and bring forward the theory that the coal-measures 

 of Commentry are old lacustrine deltas. 



Specially conceived to explain the phenomena of this 

 area, M. Fayol's theory has naturally been extended to the 

 rest of Central France, and to coal-iields in general. " I 

 propose to show," he says, " that coal-measures are 

 deposits formed by water-courses at their entrance into 

 lakes, or into the sea." And whatever may be thought of 

 this extension of his ^dews, it must be admitted that if 

 any case can be proved by scientific reasoning, experiments, 

 and observation, M. Fayol has proved his for the coal-field 

 of Commentry. 



Every phenomena exhibited by these coal-measures can, 

 he claims, be satisfactorily explained on his theory, and 

 imitated in an artificial delta. On the other hand, he 

 points out many facts which the rival theory utterly fails 

 to accoimt for. 



The first section of the second part of his book is the 

 record of the examination of the rocks of the coal-measm-es. 

 It is remarkable for its minuteness, and the amount of 

 labour it iniphes. The description is illustrated by 

 numerous and elaborate plans and sections, which serve 

 admirably their purpose of elucidating the text. 



The coal-basin of Commentry is one of those numerous 

 isolated formations scattered over the granitic and gneissic 

 plateau of Central France. It is some 5^ mUes by 2 in 



