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stances. It is evident from what we know of putrefactive processes that these changes may take place in two perfectly different ways. In the one case we have the oxidation taking place directly, all the nascent substances being satisfied by the oxygen. of the air and the splitting up of the organic matter being carried on by aerobic organism. In such a process of oxidation, which takes place in the porous soil well supplied with air and moisture, and also in water which is from time to time well saturated with oxygen, it will be found that little or no putrefactive odor is developed. The marsh gas, the sulphuretted hydrogen and other similar substances as they are set free rapidly combine with oxygen to form sulphuric acid, carbonic acid and water, and the nitrogenous substances in a similar fashion combining to form nitrous and nitric acids. In the soil these acids combine with the various basic substances, lime, magnesia and the like, are thus rapidly removed and the way is left clear for the formation of fresh batches of the same substances. In anaerobic putrefaction, on the other hand, the process does not go in this unobtrusive fashion, the anaerobic organisms having, as it were, to wrest their oxygen from the organic molecules because there is no free oxygen present, set up a much greater disturbance and the products of the decomposition such as sulphuretted hydrogen, marsh gas and ammonia, are thrown off in an unoxodized condition and in the free form (i. e., they are no longer in a nascent condition) they remain comparatively stable, and give rise to the putrefactive odors so characteristic of rapid anaerobic putrefaction.

"THE BLACKWALL TUNNEL."

By DAVID HAY, M. Inst. C. E., and Maurice Fitzmaurice, B. E., M. Inst. C. E. (Abstract from Proceedings Institution of Civil Engineer [England] Vol. CXXX, 1897.)

"DRIVING THE TUNNEL."

As it was of the greatest importance that the shield should be started to work with as little delay as possible, it was decided to erect it on the surface and lower it down No. 4 shaft as soon as the water-tight floor at the bottom should be completed. The weight of the shield without the hydraulic rams, etc., was about 220 tons, and it was decided to float it down; it was therefore built in a dry dock, forming part of the cut-and-cover trench adjoining the shaft. The ends were close planked and calked upon completion, and on a portion of the side of the shaft next the trench being removed, the shaft and trench were filled with water. The shield floated on the depth of water in the trench reaching about 17 feet; it was then towed into the shaft, and, as the water was pumped out, it sank until it reached the timber cradle previously prepared for it at the bottom.

After the shield had been placed in the tunnel opening of No. 4 cassion (in which cast iron guides had been bolted to ensure

true line and level being followed) a portion of the cast-iron lining extending to the other side of the shaft, was temporarily built up behind the shield to form an abutment for the hydraulic rams in driving the shield forward. It now became necessary to remove the plug from the tunnel opening; in doing so a commencement was made at the bottom, and as the girders carrying the bottom outside plates were removed the latter were temporarily strutted to the shield. Clay, chiefly in bags, was then built against the plates to support the face when they should be taken out. second row of plates being similarly dealt with, a sufficient height was obtained to draw out the bottom row by means of a tackle or union screw; the same process was continued with the other plates until the whole of the plug was removed and replaced by a wall of clay, through which the shield was driven into the face beyond. This method of removing the plug refers more particularly to that adopted in gravel, etc.; when the face consisted of clay such extreme care was not necessary. The ground in front of the plug was sometimes grouted with cement before the plug was removed. The strata on starting from No. 4 shaft consisted of I foot of sand at the bottom overlaid by 25 feet of London clay with about I foot of ballast showing at the top. The latter, as previousiy stated, had been drained to a large extent by the pumps for the adjacent "cut-and-cover" work, and, as it was kown that on account of the gradient of the tunnel the ballast would soon disappear, it was decided not to use compressed air at the outset, but to drive a top heading to deal with the gravel and water. The water was strongly impregnated with creosote, oil, etc., from a tar distillery above, and considerable pain and inconvenience was felt by the men through inflamed eyes, and burnt hands and arms; this trouble, however, passed away as soon as the clay was sufficiently thick to cut the water off, after which the top heading was discontinued.

At first progress was somewhat slow, only 125 feet being driven in the first two months, but after the gravel disappeared and the top heading was discontinued, better progress was made, an average length of 25 feet being completed per week. An accident, however, soon after happened to the shield which caused some delay. At the base of the London clay, and in the sand immediately below it, large pieces of rock were embedded and considerable damage was caused to the cutting-edge by driving against them. This was first discovered after fifty-four rings had been erected, and, although great care was exercised in clearing the excavation in front of the up-turned part of the cutting-edge, the damage continued to increase, and, after another twenty-six rings had been erected, the shield was found to be unworkable. As it was not practicable to repair it in its then position, it was decided to construct a concrete cradle for it to slide upon. A timbered heading, 19 feet wide, was therefore driven and kept about 50 feet. in advance of the shield, so that the concrete should have time to become hard before the shield came upon it. During the driving

of this heading trouble was again experienced from water in the ballast above finding its way through cracks in the clay, and the top heading was accordingly recommenced, so as to intercept the water and carry it through the shield. This method of working was continued until No. 3 shaft was reached, where the repairs to the shield were effected.

Until about 490 feet had been driven towards No. 3 shaft no great quantity of water was met with, but at that point a large volume suddenly broke into the bottom heading. Considerable difficulty was then being experienced in sinking No. 3 shaft, and the water which broke into the heading undoubtedly came from the ballast, and found its way either down the side of the shaft or through the cracks in the clay which had been caused by the numerous blows. As the shield was then only 67 feet from the shaft (the bottom of which was to be 15 feet below the invert of the tunnel) it was deemed prudent to suspend any further tunnelling operations until the shaft was sunk to its full depth. Meanwhile No. 1 bulkhead was built. It was constructed of concrete 121⁄2 feet thick, having two locks at the level of the temporary tramway, fitted with rubber-faced doors. Two sets of outlet and inlet-cocks were provided, one, 21⁄2 inches in diameter, for use when the locks contained materials only, and the other, 11⁄2 inch in diameter, when men were passing through. The various pipes for compressed air, hydraulic pressure, blow-out pipes, etc., were built in as shown. The inner, or pressure, side of the bulkhead was rendered with cement, and any small spaces between the brickwork and the tunnelling, caused by settlement, were grouted up through pipes built in for the purpose, and by these means no difficulty was found in making the wall air-tight. In the three bulkheads which were afterwards constructed for other portions of the work, brickwork was substituted for concrete as being more easily removable, and a smaller lock, 3 feet wide, called an emergency lock, built in near the top of the tunnel, was added. No. 3 shaft having been sunk to its full depth, tunnelling operations were resumed as soon as the bulkhead was completed, the remaining length of tunnel from this point to the shaft being driven under compressed air.

A fire which occurred in the top heading on this portion of the work caused considerable anxiety. It was feared that the escaping compressed air might carry the flames through the ground saturated with very inflammable material to the distillery above, in which case a serious conflagration would have resulted. Happily a good supply of water was at hand, and the fire was extinguished before any such accident happened.

IRON AND STEEL PLATES AND FORGINGS USED IN SHIPBUILDING COMPARED AND CONTRASTED.

By M. W. AISBITT.

(From the Iron and Coal Trade Review [Englan 1] Feb. 18, 1838.)

At a meeting of the South Staffordshire Institute of Iron and Steel Works Managers held last Saturday, Mr. J. W. Hall (president) in the chair, a paper was read by Mr. M. W. Aisbitt (Cardiff) with the above title. He remarked that the relative positions of iron and steel as regards plates, angles, and other sections used in shipbuilding at the present day, as compared with twenty years ago, were entirely reversed. In 1878, 90 per cent of the total material used was iron, whereas at the present time 90 per cent of the total material used was steel. Since the introduction of iron plates there had never been instituted a regular system of testing them, as had been adopted in the case of steel plates; and this was much to be regretted, as, owing to the steel plates from their first production having been invariably tested for tensile strength and elongation, they had proved more. reliable. In order to induce shipbuilders and shipowners to adopt steel in the place of iron, the various registries for the classification of steamers and sailing vessels agreed to adopt a reduced scantling to the extent of 25 per cent in the case of steel plates. This, in the case of a large cargo steamer, carrying 4.000 tons of cargo, and requiring 1,200 tons of iron material as against 900 tons of steel, was naturally a matter of serious moment to shipowners, as it was not merely the saving of 300 tons of iron, at so much per ton, but an additional carrying capacity of 300 tons to the steel steamer on the same draft, power, expense, etc. Hence they would readily see the reason why steel so quickly replaced iron. But after some years' experience it was found that this reduction of 25 per cent was considerably too much in many cases. Another reason for steel being preferred to iron, even in the case where it was introduced of the same scantling, was its ductility, and hence the possibility of bending it cold to any required shape. In the case of strandings, groundings, etc., he had generally found that of two given vessels which struck rocks, one steel and one iron, the iron one would cost considerably less to repa,r than the steel one. The iron, being of less tensile strength than steel, broke off short, locating the damage to a small area, and by so by breaking would generally allow the water to enter the vessel and keep her at rest until proper means, if possible, were adopted to float her. On the other hand, the steel plates being of flexible nature, would not break but buckle between the various ribs, and probably allow the vessel to float about, striking the rocks over the whole length of her bottom before measures

could be taken to prevent her from doing so. Steel had the advantage over iron in being to a great extent homogeneous, and, in the case of oil steamers, more impervious to the penetrating effects of petroleum oil. Also, beyond the great advantage of ductility in working, it was found possible to produce plates and angles of a much larger section and dimensions than ever thought possible before. This added to the strength of a vessel by dispensing with a large number of butts or joints, and also decreased the cost by the less number of rivets, in some instances amounting to many thousands. His own impression was that a combination of steel and iron would be most advantageous for shipbuilding purposes, as the reduction previously spoken of namely, 25 per cent, was rapidly disappearing; and, considering the extra labor that the material of a cargo steamship had now to withstand as against that of one built, say, fifteen years since, amounting to about 20 per cent of the actual deadweight carrying capacity, this decrease was likely to proceed still further. The paper was illustrated by lantern views.

In the discussion which followed, the Chairman said that district could claim some distinction in connection with the history of modern shipbuilding, seeing that the first iron boat was made at Bradley, near Bilston, by John Wilkinson. At the present time the district supplied large quantities of material to shipyards, and about half the anchors and cables of the world. The mysterious fractures in steel of which they used to hear a good deal were traced to the fact that it had been manipulated by men accustomed to use iron. Under existing circumstances he thought the registries acted wisely in limiting the tensile strength of the material. The time was coming when in this country the use of higher-tensile steel for shipbuilding would be inevitable, and when the same system of building in position as was adopted in the case of boilers and bridges would be employed. High-carbon steel was coming to the front very rapidly, and the British Admiralty were specifying considerably higher strengths for their forgings and castings than they did years ago. Mr. T. Turner said. the ideal construction in shipbuilding, as in boiler or bridge construction, would be to use material of all one kind. If every part of the ship was made of the same material of the same tensile strength, the difficulties of elongation and unequal expansion would be prevented. Mr. Le Neve Foster thought there should be the same rigid tests for steel as for iron. Mr. Ashton was of opinion that the failure of iron for shipbuilding was principally due to bad workmanship in the manufacture, and that if a proper list of tests were laid down the iron manufacturers could produce plates, both in the matter of homogeneousness and tensile strength, to meet all the requirements equally as well as steel.

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