the times when the ventilation was bad were very infrequent. With regard to the proposed plan of laying a pipe of 8 in. diameter from end to end of the tunnel, supplied with compressed air at a pressure of 5 atmospheres, or 6 atmospheres, to be discharged as required by stop-cocks--he should say the effect of the compressed air in issuing would not only be that of diluting the vitiated air, as stated by Major Beaumont, but that it would have the further important effect of absorbing a large amount of heat, thus reducing the temperature in the tunnel and hastening the condensation of the watery vapour. With respect to gradients, it had been suggested during the discussion that an uniform gradient should have been adopted throughout, and that thus so steep an ascent as 1 in 45-as there was now from the Modane end to the summit, or say the middle of the tunnel-would have been avoided. Without entering into details, it might be stated that it was impracticable to make an entrance to the tunnel at a higher elevation on the French side, and inexpedient to select an entrance at a lower elevation on the Italian side; the difference in level between the two entrances was 435 ft., which, equally divided over the whole length of the tunnel-7·6 miles—represented a gradient of 1 in 92. To have adopted that gradient would have required the driving from the Italian end to have been on the dip,' or downhill, which under the circumstances, and with a probability of meeting with large quantities of water, was a course no prudent engineer would adopt, or would even entertain. The plan determined upon of driving from the Italian end on a nearly level line was the best. The actual gradient from that end to the summit was only about 1 in 2000, or just sufficient to allow water to run; and the gradient from Modane to the summit was therefore a steep one, for in that distance the original difference in level had to be gained-435 ft.-and in addition the 10 ft. risen in the portion driven from Bardonnèche. With regard to the alleged cost to the Italian Government— namely, £206 per mètre-the work was sublet to Messrs. Sommeiller and Grattoni at £184 per mètre, and conjectures were made— though they could not be confirmed from official sources-that the actual cost to the contractors was a little over £80 per mètre. When the contract was let, there remained 4,300 mètres to be executed, on which a profit was made by the contractors of about £100 per mètre. Under such circumstances it might be imagined that the employés of the contractors were not willing to afford minute details of cost. In consequence, Mr. Sopwith was unable to obtain such full information as he should otherwise have desired with respect to the prices of the different sorts or classes of work. The engineers were uniformly attentive and polite to those who visited the tunnel, and gave ably and readily their experience on technical and scientific points, but they were not disposed to enter into details of cost. In defence of his statement that tunnels of this kind might be driven for £100 per mètre, he would urge that not only did he believe this tunnel to have cost the contractors less than £100 per mètre, but that the execution of the St. Gothard tunnel, which was longer, had been let for the whole length at £120 per mètre, and it was expected a large profit would be made out of the contract. The temperature in the tunnel was a very important matter, although he had only referred to it incidentally in the Paper. It would be remembered that he had stated his views as regarded the probable temperature in the following terms:-" Assuming, for the moment, that the conditions as to temperature at Mont Cenis are not materially altered by its elevation above sea level, and taking a mean of 2° Fahrenheit as the increase of temperature due to each 100 ft., the temperature in the tunnel should be 100° Fahrenheit higher than what would be observed at the bottom of a pit sunk from the summit of the section to a depth of about 80 ft." 291 He might say his reason for believing that at the bottom of a pit sunk at the top of the mountain, the temperature would be 50° Fahrenheit or 60° Fahrenheit, was derived from the theory which had been so lucidly explained by Sir W. Armstrong. Mr. Sopwith might have been wrong in assuming the depth of the supposed pit to be so little as 80 ft., but it was unimportant for his argument whether the depth of the supposed pit were 100 ft. or 200 ft., it was only necessary to suppose it to be so deep as to be beyond atmospheric influences. He had no hesitation in stating his belief that in a pit of the last named depth a temperature of 50° Fahrenheit or 60° Fahrenheit would be found, for just before he stopped at the Mont Cenis tunnel, he had been on professional business in Italy, inspecting mines about 8,000 ft. above the level of the sea. He did not take observations with a thermometer, but the general temperature of those mines was that which was found in ordinary mines, namely, 60° Fahrenheit to 70° Fahrenheit, and that was at an elevation very nearly equal to that of the summit of Mont Cenis tunnel. If it was granted that Sir William Armstrong's supposition of a thickening of the earth's crust where mountain ranges occurred was correct, it might still be open to 1 Vide ante, p. 4. doubt whether so small an increase in thickness, as compared with the bulk of that part of the interior of the earth-nearly the whole of it-which was supposed to be at an extremely high temperature, would account for so considerable a difference in the ratio of progressive heat as might be inferred to exist under the circumstances, as compared with that which had been observed to exist elsewhere. It was interesting to observe in the table given in the Paper of the rate of progress effected' the improved results which had been attained from year to year since 1863, at which time a great advance had been made on the results formerly obtained by hand labour. He might also add that the machinery was in such good working order that by many it was supposed the maximum of speed which could be attained by machine tunneling had been reached. The number of mètres driven in 1870 was more than double that driven in 1863, and so much confidence had been inspired by the results of machine tunneling that contractors had been found, as stated by Major Beaumont, who had undertaken, under heavy penalties, to execute the St. Gothard tunnel in such a time as would require an annual progress of 2,100 mètres, which was 25 per cent. more than the best results obtained in the best year at Mont Cenis. By the completion of the Mont Cenis tunnel the following facts, once vigorously discussed, had been satisfactorily proved: 1. That geodetical operations in mountain ranges, under circumstances of peculiar difficulty, could be performed with sufficient accuracy to insure the setting off, in the same axis and plane, the line of any tunnel which was likely to be required. 2. That an elevation of 5,000 ft. over the line of a tunnel in mountain ranges was not accompanied by an excessive increase of temperature in the tunnel, either whilst in progress or after completion. 3. That the service of removing 'déblais' from the advanced gallery at a distance of 4 miles from the entrance could be effected practically with the same regularity and despatch as at shorter distances. 4. That ventilation during the execution of the works of a tunnel exceeding 7 miles in length could be provided without sinking ventilating shafts. 1 Vide ante, p. 6. [1872-73. N.S.] D 5. That by the aid of machinery a tunnel in hard rock could be driven in less than a quarter of the time which would be required for its execution by hand labour. With respect to the small number of perforators worked in comparison with the number in the works, in repair or in reserve, Mr. Sopwith could only say, that he had stated the facts as he had found them. In compiling from his notes he had been anxious to avoid any deviation from them, founded on what ought to be, or might be, the result with increased experience. Whether the numerous changes which had been made in the machines from time to time had anything to do with the large proportion kept in stock he would leave to conjecture, but he believed himself that they had. With regard to the surface labour, it had also been a wonder to him that so large a number of men should have been employed as compared with those working inside the tunnel. He believed the returns given him were correct, as they had been taken from the pay sheets of the contractors. The only explanation he could give upon that point was, that it was possible to employ a large number of men in the south of Europe without getting a remarkable quantity of work done. With respect to the section of the tunnel shown in Fig. 6 (Plate 9), alluded to by Mr. Bewick, Mr. Sopwith had omitted to state that pillars of rock, 6 ft. or 8 ft. long, were left in, and that balks of timber were stretched from one pillar to the other. To have shown the pillars would only have confused the drawing. The long length of pipe which conveyed the compressed air was the principal reason why the condensed water -frequently a source of difficulty when compressed air was useddid not reach the perforators in large quantities at Mont Cenis. In the case of the Mont Cenis tunnel, the air was prepared at a low elevation, and used at a higher elevation, whereas the reverse was the case in mines. April 1, 1873. T. HAWKSLEY, President, The following Candidates were balloted for and duly elected :CHARLES HENRY BELOE, as a Member; CHARLES BENJAMIN BRAHAM, Stud. Inst. C.E., RICHARD DAVIS, FRANK FITZJAMES, FREDERICK WILLIAM HARTLEY, THOMAS FLETCHER HARVEY, JAMES WOODWARD HILL, ROBERT JOSEPH HUTTON, THOMAS FARMER PARKES, Stud. Inst. C.E., ALBERT WOODWARD PARRY, FREDERICK MANTELL PRATT, JAMES SAMUEL STATTER, and CHARLES EDWIN WARE, as Associates. It was announced that the Council, acting under the provisions of Sect. III., Cl. VII., of the Bye-Laws, had transferred ELIOTT SCARLETT CURREY and THOMAS WEATHERBURN DODDS from the class of Associates to that of Members. Also, that the following Candidates, having been duly recommended, had been admitted by the Council, under the provisions of Sect. IV. of the Bye-Laws, as Students of the Institution:-FRANCISCO CORREIA DE MESQUITA CARDOZO, JAMES MILN SMALL, ALEXANDER STARK, BARNABAS JAMES THOMAS, CHARLES HORACE WILSON, and JOHN PALMER WOODCOCK. The discussion upon the Paper, No. 1,340, "The Mont Cenis Tunnel," by Mr. T. Sopwith, jun., was resumed and concluded. |