184S.J 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



317 



of metal m which a quantity of mercury was placed, and a rotatory motion 

 was given to the disc, so that the mirror assumed a concave form, giving a 

 paraboUc surface, of the actual form required to reflect bodies ; by which 

 means a mirror was constructed in one instant of time. This appeared so 

 impracticable to that society, that thev did not venture to give anv account 

 of it in their memoirs. 



Photographic Discoveries. 

 Sir David Brewster said, he was requested to communicate an account of 

 some remarkable facts connected with the theory of photography. A new 

 process of producing photographic impressions had been discovered bv Dr 

 Moser, of Konigsberg ; and an account of the discovery had been brought to 

 this country by Professor Bessel, who received it from the discoverer liimself 

 The subject was most important, and it would have been a great misfortune 

 '^, r?, '' ^^'^'''"' '""' ^^'^ "i'liout having been made acquainted with it 

 The foUowing were the general facts connected with it :— A black plate of 

 horn, or agate, is placed below a polished surface of silver, at the distance of 

 one-twentieth of an mch, and remains there for ten minutes. The surface 

 Of the silver receives an impression of the figures, writing, or crest, which 

 may be cut upon the agate, or horn. The figures, ice. do not appear on the 

 silver at the expu-ation of the ten minutes, but are rendered visible by ex- 

 posing the silver plate to vapour, either of amber, water, mercurv, or any 

 other fluid. He (Sir David Brewster) had heard Professor Bessel' say, that 

 the vapours of different fluids were analogous to the different coloured rays 

 of the spectum; that the different fluids had different effects, corresponding 

 to those of the spectrum ; and that they could, in consequence of such cor- 

 respondence, produce a red, blue, or violet colour. The image of the cameo 

 oiscnra might be projected on any surface— glass, silver, or the smooth 

 leather cover of the book— without any previous preparation ; and the effects 

 would be the same as those produced on a silver plate covered with iodine 

 Frofessor Bessel did not pretend that this discoverv was perfect. He also 

 mentioned what was far more important than all this, in a scientific point of 

 view— that Professor Moser had discovered the existence of latent light— of 

 hght residing in bodies, and exhibiting its action after having lain in those 

 bodies for we know not how many years. If it were an actual physical fact 

 that latent light did exist— if light took its residence permanently in a body 

 —if, having motion, it stood still, and did not shine— if it did not trouble 

 the ether m any way, was its motion extinguished ? and, if so, how was it to 

 be put in motion again ? This idea of latent light corresponded with New- 

 ton s opinion, that light entered the surface of charcoal, and never was 

 brought out again ; but Moser had discovered a mode of bringins it out 

 again. ° ° 



.X. ^^ YH''*"^-"™"'"" " " "*^^ y" *"y account that he has discovered 

 J', . \-~r^Y. ^^""^ Brewster : " He conceives he can prove the existence 

 of latent hght. 



Professor Bessell said, he scarcely had time to obtain from Professor Moser 

 aU the particulars of his discovery ; but he (Professor Bessel) had seen a plate 

 of silver representing the image on a black agate cameo. This photographic 

 image was very good, and looked as weU as any of those Ukenesses taken bv 

 Talbot or Daguerre. Professor Moser said he had produced those likenesses 

 even at midnight, without using any light. The exclusion of hght made no 

 difference whatever in the phenomena. 



Sir David Brewster thought the image thus produced might have been 

 as perfect probably as the first specimens of daguerreotype. He had alluded 

 to charcoal absorbing the rays of hght— there was another analogous phe- 

 nomenon ; if a tube were filled with nitrous acid gas, and rays of light passed 

 through It by a lens, the fluid would almost lose its yellow colour, and be- 

 come so white, that every object might be seen through it; but if it were 

 exposed to a higher temperature, it became gradually vellow, then oranee, 

 &aa at length it would become of such a deep red colour, that the liRht of 

 the meridian sun could not pass through it-the gaseous body standing like 

 a column of charcoal before the spectator. By allowing the gas to cool 

 down. It gradually becomes transparent, passing backward through the same 



tTl y t ?*"'^ f"" ^'■°'° "'^ ^^*'i *"''' 'f ^^"^ be such a thing as 

 latent hght, it must have acted on the particles of the gas. 



On the Pressure of Earth against Walls. By C. W. Buck, C. E. 

 Mr Buck thought it desirable to investigate the resistance offered by 

 earth to displacement, when a rigid vertical plane is thrust against it-as 

 Tn fhtf. r °f ';"''g^\^« '""de, for economy, to depend for stabihty 

 on the resistance of the earth behind them. In cases of this sort, the ten- 

 dency of the pressure is manifestly to cause a rupture of the earth, and force 

 a wedge of It upwards; and it is curious to observe that the angle at which 

 this rupture takes place (the angle of least resistance) is the same as that 

 given for the maximum thrust by Provy-that is, half the complement of 

 ^L^fl^ of repose. To estimate the horizontal thrust against the waU 

 below the spring of the arch, when this arch is sprune Irom the top of the 

 wa^l, and when he entire height behind the wall is filled up with earth to 



h. Inf , }^ ? A'u ' '°°l °^ **" "''■' "'"^ "« t'^" '"''^^es of ^"th to 

 the ton nf, !,",'*• ^^' r''^' "'^^'"^"^ acting against that portion from 

 fitrlT?/ t' ^t "'' *,° *'" '°''u"^ ">' "''' ' ""'^^ 2nd, The quadrilateral 

 ngureleft when this wedge was subtracted from the entire wedge from the 



foot of the wall to the top. It is the horizontal thrust of this latter nor- 

 tion which ,s to be considered; and this horizontal thrust is equal to the 

 honzontal thrust of the entire wedge ,«m,« the horizontal thL of hi 

 wedge above the top of the wall m,«,« the friction of the base or line cf 

 "P r/- ^,^"T "■•^''g'^- This can be found easily for anv part "ul^ 



twU.h ',1''^"';"''^' '° ^""^ ''"^ P^^P" ''^'g''» f^«°> «l'i'--l^ to spring 

 the arch, so that the friction of the Hues of rupture of the two wedeef 

 shall nentrdize the honzontal thrust against the wall below the arch. This 



;LTn„f°ih "r"" f'T ^™" ""='^^^' °f'he roof of the arch t» 

 he top of he wa 1 is equal to the entire depth, from the top of arch to the 

 foot of wall mult.phed by the square root of the tangent of angle of maxi- 

 mum thrust. Following out this result for fluids, where the friction is 

 »«//»«j, he arch should spring from the bottom of the wall ; and this Mr 

 Buck called the "Tunnelling Equation," because it shows what thickness of 

 roof IS suflicient to neutralize the pressure of the sides, and points out how- 

 far It IS necessary to cut the ground open, curb it, and cover it again when 

 there is not suflicient thickness of roof for ordinarj- tunnelling. It shows 

 also, how strong to make the side walls, and what form to give them to 

 resist the lateral thrust. The investigation further proved the douRer of the 

 conimon practice in sinking shafts, of sinking to a considerable depth before 

 curbmg, or securing with brick-work, from the supposition tbat the earth 

 must needs stand of itself; and, therefore, that it will answer as well to 

 begm the curbing at some inferior point ; whereas the reverse is true— that 

 If the top be well secured, the part below will not move. These remarks 

 however, are based on the supposition that the earth is homogeneous Mr' 

 buck was proceeding to read over the mathematical formula of his investi- 

 gation, when — 



The President thought the subject was too analvtical to be interestine to 

 the Section.— Professor Moseley stated that Poncelet had lately investigated 

 all the conditions of subjects tnken up in Mr. Buck's paper, and given for- 

 mula; and tables for their application, by practical men, except the one for 

 tunnelling Poncelefs paper was published in the Memoirs of the Academy. 



Sir Mark J. Brunei thought it dangerous for engineers to place reliance 

 upon tormula: which applied only to homogeneous ground. In the Thames 

 Tunnel he had found ten or twelve different strata in a depth of eight feet • and 

 from the constant fluctuation of the weight above, and the fluid portion of 

 some part of the ground, theoretical inquiries would have been useless in 

 determining the manner of his proceeding.— Mr. Buck observed, in answer, 

 that even in the case of fluid ground, theorv assisted in determininR the re- 

 sistance necessary. Taking the London clav as an example, the friction in 

 the line of maximum thrust was ^ of the weight; and by comparing the 

 specific gravity of this clay with water, he found that where, bv the infiltra- 

 tion of water, this friction was reduced to f^, the pressure was equal to 

 hydrauhc pressure, and when the friction was reduced to 0, it was double 

 hydraulic pressure. 



Sir John Robison made a short communication on the subject of a mode 

 of making tesselated tiles and other objects hy the compression of dru powder 

 of clay, by Messrs. Minton cj- Co., of Sroke-upm-Trent. Sir John stated, 

 that the advantages of the invention were the perfect truth of the tiles. When 

 formed of wet clay, they almost invariably warped and became crooked In 

 this invention, a mould was filled with the dry powdered clay, which was 

 subjected to heavy pressure, which caused it to cohere suflicientlv to bear 

 being placed in the kiln, where it was burned in the usual manner Some 

 specimens of tesselated tUes, and also of roofing tiles of a new construction, 

 much lighter than ordinary tiles, and by which the capiUary action of water 

 under the edges of the tUes, was prevented, were exhibited. The tiles were' 

 exceedingly accurate in point of form, and appeared extremely weU adapted 

 for the purposes for which they were intended. 



Mr. Brockedon exhibited specimens of his patmt India rtiiber stoppers for 

 bottles, explaining the late improvements in the construction of the cores on 

 which the India rubber is spread. The present cores, he said, were made of 

 cotton twisted into strands, &c., by means of a machine, which he explained 

 by a diagram, the cylindrical rope now consisted of several strands of tightly 

 twisted cotton, lapped with flax thread, and laid together longitudinally, 

 loose fine cotton rovings being placed between them ; the entire was then 

 lapped in a cyhndrical form with flax thread, attaining by this method the 

 advantages of perfect roundness and firmness ; they also gave sufficient hold 

 to the corkscrew, and bore the heating process well. These stoppers would 

 slide on glass when wet, but not when dry, (although there was no cohesion 

 m this latter state,) so that the bottler, by slightly wetting these stoppers 

 with the liquor which he was botthng, could easily insert them ; and when 

 this slight film of moisture mas dried up, the stopper required considerable 

 force to withdraw it. 



Mr. Louis Schwabe made a short communication on theformation of arti- 

 ficial threads of glass, for the manufacture of ornamental damasks, some very 

 beautiful samples of which he exhibited to the section. Mr. Schwabe stated, 

 that he had tried various kinds of glass, and found that that which was best 

 adapted to the purpose was common window glass ; the samples before the 

 meeting having been made from the broken windows of his own estabUsb- 

 ment. There was one peculiarity in the matter which was worthy of re- 

 mark, that the minute fibres of glass drawn from the lamp were always of 

 the same shape as the bar of glass from which they were drawn. If that 

 was round, the thread was round, if square or flat, the form of the thread 

 varied accordingly; and, as a flat thread reflected the light better, and con- 



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