62 



NA rURE 



{May 24, 1877 



de M. Steenstrup. Si vous consultez le Record of Zoological 

 Literature ^owx 1S69 (vol. vi.), vous y trouverez, p. 139 : — 



" Selache mi.xima. — A detailed description and figure of an 

 example from the coast of Portugal is given by M. Capello 

 under the name of Cetorhinus hlainvillii, Jour. Ac. Sc. Lisbon, 

 No. vii,, p. 233." 



Je vous envoie par la poste le No. vii. du Jour, des Sc. de 

 Lisboitne. J. N. Barbosa Bocage 



Gold in Carboniferous Conglomerate 



Many of your readers are aware that the fact of the occurrence 

 of gold in Lower Carboniferous conglomerate as in New South 

 Wales is not at all new. The Gay's River (iold Field of Nova 

 Scotia, where the gold occurs in Lower Carboniferous conglo- 

 merate resting on the edges of Cambrian slates having small 

 veins of auriferous quartz, was first pointed out by Prof. Hartt 

 and elaborated by myself in a paper communicated to the Nova 

 Scotian Institute of Natural Science in 1S66. In Dawson's 

 " Accadian Geology," of 1S68, the same fact is referred to; also 

 In Seluria, Prof. R. Jones received specimens of the conglo- 

 merate from me in Paris, 1S67, to satisfy Sir R. J. Murchison 

 of the fact. In the collection of ores and concrete minerals sent 

 by H. S. Poole, Esq., Government Inspector of Mines to the 

 Centennial Exhibition, in my charge, was a very instructive 

 specimen of slate with a little of the conglomerate attached, 

 having a beautiful display of gold. This was much admired. 

 The conglomerate of Gay's River is overlaid by limestone with 

 Lower Carboniferous fauna and gypsums. The conglomerate is 

 worked still with good results. D. IIoneyman 



Provincial Museum, Halifax, Nova Scotia 



Japanese Mirrors 



A SHORT time ago a friend showed me a curious effect, which 

 I had previously heard of, but had never seen. The ladies of 

 Japan use, in making their toilet, a small round mirror about jlj 

 to -5 inch in thickness, made of a 1-ind of speculum metal, biighily 

 polished and coated with mercury. At the back there are usually 

 various devices, Japanese or Chinese written characters, badges, 

 &<;. , standing in strong relief, and brightly polished like the front 

 surface. Now if the direct rays of the sun are allowed to fall 

 upon the front of the mirror and are then reflected on to a screen, 

 in a great many cases, though not in all, the figures at the back 

 will appear to shine through the substance of the mirror as 

 bright lines upon a moderately bright ground. 



I have since tried several miiTors as sold in the shops, and in 

 most cases the appearance described has been observed with 

 more or less distinctness. 



I have been unable to find a satisfactory explanation of this 

 fact, but on considering the mode of manufacture I was led to 

 suppose that the pressure to which the mirror was subjected 

 during polishing, and which is greatest on the parts in relief, was 

 concerned in the production of the figuies. On putting this to 

 the test by tubbing the back of the mirror with a blunt pointed 

 instrument, and permitting the rays of the sun to be reflected 

 from the front surface, a bright line appeared in the image cor- 

 responding to the position of the part rubbed. This experiment 

 is quite easy to repeat, a scratch with a knife or with any other 

 hard body is sufficient. It would seem as if the pressure upon 

 the back during polishing caused some change in the reflecting 

 surface corresponding to the raised parts whereby the amount of 

 light reflected was greater ; or supposing that of tlie light which 

 falls upon ihe surface, a part is absorbed and the rest reflected, 

 those parts corresponding to the raised portions on the back are 

 altered by the pressure in such a way that less is absorbed, and 

 therefore a bright image appears. This, of course, is not an 

 explanation of the phenomenon, but I put it forward as perhaps 

 indicating the direction in which a true explanation may be 

 looked for. 



The following account of the manufacture of the Japanese 

 mirrors is taken from a paper by Dr. Geerts, read before the 

 Asiatic Society of Japan, and appearing in their Transactions for 

 1875-76, p. 39 ■■— 



'■ For preparing the mould, which consists of two halves, put 

 together with tlieir concave surfaces, the workman first powders 

 a kind of rough plastic clay, and mixes this with levigated 

 powder of a blackish " tuff-stone " and a little charcoal powder 

 and water, till the paste is plastic and suitable for being moulded. 

 It is then roughly formed by the aid of a wooden frame into square 

 or round cakes ; the surface of the latter is covered with a levi- 



gated half-liquid mixture of powdered " chainotle" (old crucibles 

 which have served for melting bronze or copper) and water. 

 Thus well prepared, the blackish paste in the frame receives the 

 concave designs by the aid of woodcuts, cut in relief. The two 

 halves of the mould are put together in the frame and drtefl. 

 Several of these flat moulds are then placed in a melting box 

 made of clay and "chamotte." This box has on the fop an 

 opening, into which the liquid bronze is poured, affei itlia$been 

 melted in small fire-proof clay crucibles. The liquid metal 

 naturally fills all openings inside the box, and consequently also 

 the cavities of the moulds. For mirrors of first quality the fol- 

 lowing metal mixture is used in one of the largest mirror 

 foundries in Kioto : — 



Lead ... 5 parts. 



Tin 15 ,, 



Copper ... ... ... ... ... So ,, 



For mirrors of inferior quality is taken — 

 Lead 



Natural sulphide of lead and antimony 

 Copper ... ... ... ... ... So 



" After being cooled the melting-box and moulds are crushed 

 and the mirrors taken away. These are then cut, scoured, and filed 

 until the mirror is roughly finished. They are then fir^t polished 

 with a polishing powder called to-no-ld, which consists of the 

 levigated powder of a soft kind of whetstone {to-ishi) found in 

 Yamato and many other places. Secondly, the mirrors are 

 polished with a piece of charcoal and water, the charcoal of the 

 wood, lio-no-ki {Magnolia hypoleuca) being preferred as the 

 best for this purpose. When the surface of the mirror is well 

 polished it is covered with a layer of mercury amalgam, con- 

 sisting of quicksilver, tin, and a little lead. The amalgam is 

 rubbed vigorously with a piece of soft leather, which manipula- 

 tion must be continued for a long time until the excess of mercury 

 is expelled and the mirror has got a fine, bright, reflecting 

 surface." R. W. Atkinson 



University of Tokio, Japan 



THE DECENNIAL PERIOD OF MAGNETIC 

 VARIATIONS, AND OF SUN-SPOT FRE- 

 QUENCY 



A CENTURY and half ago Graham discovered that 

 the north end of a magnetic needle moved from 

 mornin;^' till afternoon towards the west, retttrnlng there- 

 after to its most easterly position in the morning again. 

 Van Swinden, who, half a century later, studied this 

 phenomenon during several years, occupied himself 

 greatly with the deviations froin the diurnal law. One of 

 these, the occurrence of the greatest westerly position 

 before noon or after 4 p.:\i., he found to happen most 

 frequently in 1776, the number of times increasing from 

 1772, and diminishing from the year of maximuifi till 

 17S0. He then asked the question whether there was 

 not a period of cigiit years. \^an Swinden's results were 

 greatly affected by imperfections of his instrument, and 

 we can only consider that tire excess of irregular days in 

 1776 was probably chiefly due to real causes. 



Though several series of magnetic observations were 

 made during the eighteenth century, and two series early 

 in this (those of Beaufoy and Arago), yet, as far as 1 can 

 discover, Kaemtz seems (in 1836) to have been the first to 

 remark that the mean value of the diurnal oscillation of 

 the magnetic needle was not constant, but varied from 

 year to year : this conclusion he founded on Cassini's 

 observations, which gave the mean oscillation 9'7i in 

 17S4, and i5''io in 1787. The illustrious Gauss drew 

 more distinct attention to the fact, for, in studying the 

 observations made at Gottingen in the years 1834 to 1837, 

 he pointed out that the mean diurnal oscillation for each 

 month in the second year was greater than that for the 

 corresponding month of the tirst year ; and that a similar 

 increase was to be found in the third year compared with 



