316 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[September, 



inent conUl be moved forward or backward at pleasure, so that, as the 

 •workmen excavated a few inches, the superincumbent pressure was imme- 

 diately sustained by the protrusion of that part of the machinery opposite 

 which the excavation was made. The ground was cleared away in front of 

 each alternate division of the moveable arches, so that six were always 

 stationary, sustaining the roof of the tunnel ; while the other six were 

 employed in making additional progress. Some of the strata overhead was 

 quite dense, while part of it was in a fluid state. The workmen never 

 advanced more than six inches at a time, and then the props were imme- 

 diatelv introduced into the vacant place, and the building-up of buttresses 

 and arches followed in the rear. During the progress of this extraordinary 

 undertaking, five eruptions of the river had taken place. One of these 

 occurred when the tunnel had been half done. This was in consequence of 

 the men having struck for an advance of wages ; and, during the delay which 

 ■was thus occasioned, a very large hole opened in the top, and the water 

 came pouring in. When this occurred, said Sir Mark, every one made a 

 hole for his stopper ; but he could get no one to make a stopper for the hole. 

 Bv that unfortunate eruption, five men were lost. However, he succeeded 

 in stopping the leakage, and in pushing forward the works, until he reached 

 the other side of the river. After surmounting many difficulties, he had 

 now the pleasure to come before the public for the first time, and say the 

 ■work was done. Many had endeavoured to discourage him, and thwart the 

 undertaking, but the Government were always exceedingly liberal ; and the 

 Duke of Wellington, from the first moment that he had seen the plan, con- 

 ceived a high opinion of its practicability, and forwarded the undertaking as 

 much as possible. A tower, 290 ft. in diameter, was erected at each side of 

 the tunnel, and carriages could he admitted through them with the greatest 

 ease. In answer to a question from Mr. Roberts, Sir M. I. Brunei said, that 

 the quantity of water percolating at present through the tunnel was about 

 60 or 70 gallons per minute, .\bout six weeks ago. 480 gallons oozed 

 through ; the quantity had since been reduced to about 00 or 70 ; and, in a 

 short time, it would be reduced to nothing. 



On the l.MPROVEMENT OF THE SpECULA OF TeLESCOPE.S. By Mt. FoX 



Talbot. 



The subject occurred to him about two years ago, when the Earl of Rosse 

 (then Lord Oxmantoun) was making much larger specula for reflecting 

 telescopes, than had ever been obtained before ; and he thought, if once we 

 had a very large and perfect speculum, it might be possible to multiply copies 

 of it by means of the electrotype— by galvanic means taking an electrotype 

 cast from the existing original at a small expense — which, if not quite so 

 perfect as the original, should be at least very fine and important instruments. 

 He had observed, that if an electrotype cast -were taken from a perfectly 

 polished surface, the cast was also perfectly polished ; so that no defect of 

 form from this cause could have an injurior.s efi'ect on the speculum. The 

 great and obvious defect was, that electrotypes being in copper, which 

 reflected liut little light, a very large speculum of copper would not reflect 

 more light than a very small one in speculum metal. He mentioned these 

 ideas to Professor Wheatstone, who said the same had occurred to him, and 

 he showed him a paper which he had drawn up some few months before, and 

 which he lent him. 



Mr. Talbot read this paper of Professor Wheatstone's, in which he sug- 

 gested the taking galvanO-phstic casts of specula in platina, palladium, 

 silver, or nickel, and for especial purposes gilding the copper ; taking care 

 that the two precipitations adhered well to each other. He suggested that 

 voltaic electricity might thus enable us to copy large specula (like those of 

 Lord Oxmantoun), at a slight expense. So that (said Mr. Talbot) the idea 

 had suggested itself independently to both of them ; but on comparing notes 

 they found differences. Though it had occurred to him (Mr. Talbot) to 

 precipitate w'.iite metals, yet he did not think that platina would ha.e a 

 sufficiently beautiful white metallic polish. Silver he rejected, because easily 

 oxidated iiy the atmosphere. Nickel he had not tried, but it appeared likely 

 to answer. Professor Wheatstone bad, however, made choice of platina, and 

 varying the quantity till he found the required proportion, he obtained a 

 mirror in platina which appealed to him (Mr. Talbot) to have quite brilliant 

 polish enough, and to he white enough to answer the purpose; and he con- 

 sidered, therefore, that Professor Wheatstone had proved that, at least in one 

 form, the specula of telescopes might be made by voltaic precipitation. His 

 own idea was, that it might be i)ossible to whiten the surface of the copper 

 without injuring the form ; and, therefore, having obtained a speculum in 

 very bright polished copjier, he exposed it to the vapour of the hydro- 

 sulphuret of ammonia, which had the property of turning copper v.hite ; 

 and it did so without, so far as he knew, in the least injuring the form of 

 the metal. A copper speculum was subject two evils; it reflected but little 

 light, aud it became easily tarnished ; but, when whitened by exposure to 

 the hydro-sulphuret of ammonia, the surface was transformed into a sul- 

 phuret of copper, and not only was a white metal obtained, but it was no 

 iongcr subject to be tarnished by the atmosphere, the sulphuret of copper 

 being a stronger chemical compound than the oxide of copper, and, con- 

 sequently, oxygen had no more effeet upon it than on jilatina. Having 

 obtained' a copper cast from a polished speculum, he, (Mr. Talbot) whitened 

 it, and transformed it into sulphuret of copper ; and, after having retained 

 it about a year, he did not percene the smallest alteraticn in any respect. 



This, therefore, appeared to him a mode by which such important results 

 for astronomers could be obtained. For the last year, perhaps, nothing 

 farther had been done, either by Professor Wheatstone or himself; but the 

 other day, being at Munich, he "(Mr. Talbot) visited Professor Steinheil, who 

 showed him his inventions, and told him he had invented a method of making 

 specula by the electrotype. It so happened, that both Professor Steinheil 

 and himself had published their respective methods about a month or six 

 weeks before, the Professor having read a communication on the subject 

 before the .\cademv of Sciences at Munich, and printed it, and he (Mr. 

 Talbot) having published his in England. Their modes were, however, 

 different, as Professor Steinheil precipitated gold upon the speculum of 

 copper; and, having precipitated a certain thickness of gold, he then pre- 

 cipitated copper on the back of the gold, to give it sufficient thickness. He 

 (Mr. Talbot) should have thought beforehand that gold would not reflect 

 light enough to be available ; but Professor Steinheil informed him he had 

 fo'und, bv careful experiment, that it reflected more light than polished steel. 

 He allowed Mr. Talbot to look through a Gregorian reflecting telescope, of 

 which the speculum was a common one, but gilded, and he found the image 

 perfectlv clear and well-defined. .\ slight tinge of yellow was thrown over 

 all the objects, but the image was perfectly clear and defined. Now it was 

 evident, that if the form of the speculum was not destroyed by gilding it, 

 much less would it be altered if formed originally of gold. If a film of gold 

 were precipitated upon a speculum of copper, we must, to a certain extent, 

 alter its figure ; but that alteration must he very small, because there was no 

 perceptible defect in the image. Professor Steinheil said, that the astronomers 

 of Germany were much pleased with his plan, and were greatly engaged in the 

 subject; and that in the coursi: of a year he should have a very large telescope, 

 furnished not only with a speculum, but also with other apparatus, voltaically 

 formed, so that telescopes might be made all from a good model, so as to in- 

 sure greater accuracy of proportions ; and in this way even very large telescopes 

 might be constructed at a comparatively trifling expense. With reference to 

 the precipitating copper on the back of the gold, the Professor had a simple 

 expedient for securing adhesion. He first precipitated gold from the cyanide 

 of gold, and he mixed with it cyanide of copper, and kept gradually increasing 

 the quantity of the latter sort; so that an alloy was precipitated which was 

 continually" increasing the copper with respect to the gold, till be had a 

 speculum whose surface was gold, and which then became an alloy, the 

 quality decreasing, till, at the bottom, it became pure copper. This was 

 important, because, without such experiments, one would not have known 

 that such results would have followed ; for some philosophers supposed that, 

 if we attempt to precipitate the salts of two metals, only one was precipi- 

 tated ; but Professor Steinheil informed him that they precipitated in union. 

 He thus obtained a speculum with a face of gold aud a back of copper. But, 

 supposing the largest, cheapest, and best speculum were obtained, the frame- 

 work of the telescope would be so gigantic, that few observers would be able 

 to use the instruinent. With a focariength of 00 to SO ft., it would be quite 

 unmanageable for any private individual. The idea occurred to him (Mr. 

 Talbot) to have a tube fixed in an invari.ihle position, and to have a perfectly 

 true plane mirror, cf a size somewhat larger than the concave speculum, 

 placed in front of the tube, with an aiierturc in the centre. This plane 

 reflector should be moveable .about its centre in any direction ; so that 

 luminous bodies, falling first upon the plane reflector, were then reflected 

 against the concave reflector, aud passed through the aperture. The only 

 motion requisite for the plane mirror would be one about its centre. ^ The 

 mechanical difliculties in the way of this plan would be far less than in the 

 common method. Professor Steinheil's idea on this point was somewhat 

 different. He (Mr. Talbot) did not think it important in what direction the 

 tube of the telescope was directed. Professor Steinheil's idea was, that it 

 should he pointed directly to the pole of the heavens, aud kept as steady as 

 possible, and that the plaue mirror shoidd have a simple motion of revolution, 

 indeed two motions, but aliout a rectangular centre. 



Remarks. — Sir William Hamilton expressed his pleasure at hearing this 

 communication, and said, the want of some such contrivance as the last had 

 become sensible to those having great command of money and power. The 

 Eari of Rosse had lately cast a metal speculum, weighing several tons, and 

 six feet in diameter, which his lordship intended should he moveable only in 

 or very near t!\e meridian, on account of the great mechanical difficulty of 

 moving so great a mass, so as to direct it at pleasure to every point of the 

 heavens, the advantages, therefore, would be great from carrying out the 

 plan last mentioned. It was clear, that its success and utility must depend 

 "■reatly on the excellence of the original. Lord Rosse had, hitherto, been 

 unriva'lled for making specula on a large scale, and for his means of carrying 

 forward this magnification of the mirror to an unparalleled extent ; aud per- 

 haps the degree of acquaintance with which that nobleman had honoured 

 him, and the recollection of what Dr. Robinson had recently said in his lord- 

 ship's name, warranted liim (Sir William) in s,aying, that Lord Rosse was 

 disposed to give every facility to persons of science, desirous to make use of 

 his instruments. 



.Mr. Isaac Ilolden said, the late Earl of Stanhope had invented and erected 

 a telescope on the plan tas to the fixity of the frame-work) which Mr. Talbot 

 recommended. The instrument was stationary, and he could sit still and 

 view more than one-half the heavens, taking in even 20" behind his head, by 

 means of an arrangement of mirrors. 



Sir D. Brewster mentioned a similar plan, proposed twenty years ago, by 

 an American gentleman, to the Royal Society of Edinburgh. There the mir- 

 ror was not vertical like Lord Stanhope's, but horizontal, consisting of a dish 



