II. The Application of the Pumping Power.-The motive power consists of a 230 horse-power compound surface-condensing engine, employed to pump from the bore-holes into a masonry tank by the engine foundations, from which tank the water is forced by the same engine to a reservoir at an elevation of 260 feet. The engine is made to work the force-pump by means of a tail rod from the low-pressure cylinder, the bore-hole pumps being worked by means of rocking levers actuated by a connecting-rod from the crosshead of the engine. There is no flywheel or rotary motion, but a very simple expedient is resorted to to enable the engine to work expansively. This steam distribution is effected by giving a peculiar bell crank form to the levers which convey motion to the well pumps. The effect of this mode of coupling the pump piston or plunger to the engine piston is to make the pump-resistance diagram so nearly approach the shape of the combined engine diagram that the weight of the moving parts of the engine is of itself, by its inertia, sufficient to equate the two diagrams. Steam Distribution.-The engine is of the receiver type, having separate expansion valves on both high- and low-pressure cylinders, adjustable by hand whilst the engine is in motion. A careful trial of the engine has been made, and as it is provided with a surfacecondenser, it was quite easy to ascertain the exact quantity of steam used by the engine by measuring the air-pump discharge, and adding that discharged from the steam-jackets. The efficiencies are as follow: 1. Engine efficiency: the proportion which the area of the actual indicatordiagrams bears to the area of the theoretical diagram for the steam admitted to the engine = '644. 2. Mechanical efficiency, or the portion of the indicated power utilised by the pumps 87 per cent. 3. Thermal efficiency, or the portion of the energy due to the fall in temperature of the steam which has been utilised by the engine = ·433. Units of work per unit of heat = 110 8. The steam-cylinders are both steam-jacketed completely-bodies and endswith steam at boiler pressure. The following summary gives the general particulars and cost of the installation: FRIDAY, AUGUST 10. The following Papers were read :— 1. At a joint meeting with Section A: (a) On Planimeters. By Professor O. HENRICI, F.R.S.-See Reports, p. 496. (b) Note on the Behaviour of a Rotating Cylinder in a Steady Current. By ARNULPH MALLOCK. (c) On the Resistance experienced by Solids moving through Fluids. By Lord KELVIN, P.R.S. (d) A Discussion on Flight. Opened by Mr. HIRAM S. MAXIM. 2. On the Strength and Plastic Extensibility of Iron and Steel. 1. For several reasons the stress-strain diagram, as autographically drawn in connection with the testing of a ductile material, does not suffice to indicate any definite relation between tensile stress and plastic strain. The stretching effect of the load is commonly measured before it is fully developed; the ordinates of the diagram represent stress per unit of original area, but cannot represent the actual unit stress, which varies in different parts of the bar; and, lastly, the elongation measured by the diagram is the elongation of the whole bar, and is the sum of the component elements of elongation, which vary still more widely in different parts of the bar. 2. Analysing the total elongation, and plotting as co-ordinates the actual intensity of stress in any short segment or element of the bar, and the actual elongation in the same element under a long-continued load, the curve takes a very different form, and is probably already familiar to those who have taken a special interest in the subject. 3. So far as the author's direct experiments have gone, the curve so traced appears to be almost identical for all parts of a fairly homogeneous bar, indicating that the plastic extensibility, or the actual extension under any given stress, is nearly the same in all segments of the bar's length, even when the ultimate elongation varies, as it often does, between 12 per cent. and 120 per cent. in the different segments. 4. Volumetric measurements of the successive segments indicate that there is no sensible telescopic shear, or internal flow of material from one segment to another (except at one point, which need not be here noticed), and justify the general application of the assumption of unchanging volume. 5. Probably, therefore, the curve may be taken to represent the variable length x assumed by each component internal element under the long-continued stress y, as well as the variable length of any cylindrical segment under the same stress, the original length of the element or segment being denoted by L. 6. It may at first sight be supposed that a bar of uniform plastic extensibility ought to draw out uniformly over its whole length; but when the curve is considered along with the mechanical conditions affecting the question, they seem sufficient to explain the observed behaviour of the bar. Beyond a certain critical point a uniform extension is almost impossible, just as the uniform compression of a long and slender strut is impossible. The formation of a short narrow neck in some part of the tie-bar is inevitable-like the buckling of the strut. 7. To illustrate these points and some others the curve, as approximately determined for a bar of mild steel, was shown in the diagrams exhibited, and its regularity suggests the existence of a definite law of extension.. As the length r increases, the resistance of the material increases continuously up to the point of fracture. Nevertheless, it is shown that, as a increases, the relations between the stretching force and the resistance pass from a condition of stable, through indifferent, to unstable equilibrium. The law of plastic extension, y=(x), as defined by the curve, fixes mathematically the occurrence of the plastic limit;" and it fixes also the 'breaking weight per square inch of original area,' which can have only one value, and is easily found by graphic construction. The breaking weight of the tiebar as thus defined depends upon the plastic extensibility, in much the same way that the breaking weight of a strut depends upon the elastic modulus. 8. Examining next the possibilities of deformation in a bar of uniform or nearly uniform extensibility, it may be seen that, as the plastic limit is approached, the slightest irregularity in section or in extensibility tends to precipitate the formation of a contracted region, and ensures that beyond that limit the further extension of the bar and the further contraction of area will be confined to the same region. For stresses below the plastic limit the probabilities of deformation may be examined by considering the relative time-rates of extension at two elements which may have been unequally stretched, and at first the tendency is theoretically in favour of preserving the cylindrical form of the bar. But beyond the plastic limit these conditions are reversed, and the tendencies are all in favour of precipitating the most rapid contraction of area at the point where any contraction already exists, and thus to pinch in still further the region where any contraction of form has begun to show itself. The observed phenomena agree very well with these deductions, and may be rationally attributed to the operation of the mechanical conditions named. 9. Going back to the yield-point, the sudden elongation which here takes place appears to be something different from plastic extension. Examined analytically, the process is found to take place at somewhat different stresses in the different segments, while in any one short element or segment it seems to be instantaneous. If the yield is arrested midway and the bar examined, it is sometimes found that the elongation has been quite completed in some segments, and not even commenced in others. This irregularity is greater than anything met with in connection with plastic extension, and may account for the variations of form observed in autographic diagrams at this point. When the process has taken place throughout, it constitutes an elongation which is almost uniform throughout the bar, and after the yield the segments fall into line and exhibit a nearly identical curve. As regards the slight curvature below the yield-point, it may be doubtful whether this indicates the commencement of yield in some short elements, or the commencement of a plastic extension in the course of which the yield occurs as a separate incident. 10. The author recognises the insufficient nature of his experimental data, and the need for further study and more refined measurements of the true curve of plastic extension; but judging from the results obtained, some formulæ are suggested as representing the probable law of plastic extension for such a ductile material. 11. In the paper some further experiments are described which were made with the object of measuring the extensions on a larger scale, and of testing some of the propositions before referred to. 3. On Tunnel Construction by means of Shield and Compressed Air, with special Reference to the Tunnel under the Thames at Blackwall. By MAURICE FITZMAURICE. Since 1892, when Mr. George F. Deacon read a paper before the British Association on the construction by means of shield and compressed air of the Vyrnwy Aqueduct Tunnel under the Mersey, other and larger tunnels have been constructed and are now in course of construction in this country by the same methods. Three tunnels have been thus completed under the Clyde just below Glasgow, and tunnels at several points in Glasgow are being made in connection with the Glasgow District Subway. A tunnel is also being constructed in Edinburgh by the same means. In all these tunnels compressed air has been used in conjunction with shields almost continuously, as the amount of water met with has been large, and it has been necessary in most cases to avoid all subsidence of the ground above as much as possible. The tunnel under the Thames at Blackwall which is being built for the London County Council under the direction of their chief engineer, Mr. A. R. Binnie, has now been under construction for more than two years, and although the greatest difficulties have probably yet to come, still an account of the present state of the work and the difficulties met with up to date will, it is hoped, not be without interest. Before dealing with the Blackwall Tunnel the author made a few remarks on previous tunnels constructed by one or both of the methods under consideration. The tunnel under the Thames between Wapping and Rotherhithe, constructed by Brunel between the years 1825 and 1842, was the first tunnel constructed by means of a shield, and its history is so well known that it will not be necessary to refer further to it. It may be noticed, however, that in a patent of Brunel's taken out in 1818 he had at that time conceived the idea of a tunnel made of cast iron with an inside brick lining, and constructed by means of a shield which had a tail lapping over the completed portion of the tunnel and shoved forward by means of hydraulic rams. Cochrane took out a patent for using compressed air in the construction of shafts and tunnels in 1833, but it was not used for the former until 1839, and was not used in any tunnel before 1872 or 1873. The use of compressed air in conjunction with a shield, and the construction of the tunnel itself with cast-iron rings, although the latter may not be so important in some cases, may be considered the key to tunnelling in loose or soft ground filled with water. The question of settlement, especially in towns, is a very important one. When pumping has to be done the water is naturally drawn down in the adjacent strata, and in addition quantities of sand often come with the water, and settlement occurs from the first or from both of these causes. When compressed air is used no pumping, of course, is necessary, and therefore there can be no settlement under that head. Probably the most fruitful cause of settlement in ordinary tunnels is caused by the fact that more ground is taken out than the tunnel actually fills, and although the utmost care is taken in supporting the ground and packing all cavities, a certain amount of settlement invariably occurs. With a shield the excavation is reduced to almost the net section of the tunnel, and therefore no settlement can take place to any appreciable extent. As regards safety in working, it is evident that when only the face of the excavation is open, and that, perhaps, only in small areas, and the water is kept back by compressed air, the maximum of safety is assured. The great advantages of constructing the tunnel of cast-iron segments are, that it is much quicker to build than anything else, and that it has its full strength as soon as it is built; and this latter is a very important matter in soft ground, which exerts a heavy pressure on the tunnel. The Tower Subway, 7 ft. 1 in. in external diameter, constructed by Mr. Peter Barlow in 1869, is interesting as being the first tunnel in which a shield shoved forward as one structure was used, and for the construction of which cast iron was adopted. It was driven through London clay, no water had to be dealt with, and no difficulties were encountered. In 1870 an experimental length of tunnel 8 ft. in external diameter was driven under Broadway, New York City, by means of a shield, to demonstrate the practicability of constructing tunnels by this method without injuring buildings by settlement. Between 1870 and 1874 tunnels of 6 ft. diameter were driven by the same means under the streets of Cincinnati for drainage purposes, and for 1 mile under Lake Erie for the supply of water to Cleveland, Ohio. The first large tunnel completed by means of shields and compressed air was the St. Clair Tunnel, finished in 1890. It was constructed with cast-iron segments of an outside diameter of 21 ft., and compressed air up to a pressure of 32 lb. was used. This tunnel was principally through soft clay, and the maximum progress in one month at one face was 382 ft. About the same time Mr. J. H. Greathead completed the City and South London Railway in London. It consists of two tunnels, made of cast-iron segments, 10 ft. 6 in. in diameter each, and 3 miles long. Compressed air was only necessary for short distances at three points, as it was principally driven through London clay. Previous to the construction of the St. Clair Tunnel compressed air had been used in the Hudson Tunnel, which still remains unfinished owing to financial difficulties. The greater portion of its length is constructed of cast-iron rings of 19 ft. 8 in. diameter. In 1891 Messrs. Pearson and Son's tender for the construction of the Blackwall Tunnel, amounting to 871,000l., was accepted by the Council, and the work was commenced in 1892. Mr. D. Hay and the author were appointed as resident engineers under Mr. A. R. Binnie, and Mr. E. W. Moir took charge of the works for the contractors. The Blackwall Tunnel is much larger than any tunnel yet constructed by the methods adopted. The outside diameter of the St. Clair Tunnel, which is the largest one at present, is 21 ft., while that at Blackwall is 27 ft. in external diameter. The following are some of the leading dimensions: The tunnel is level under the river, and the gradient on the north side is 1 in 34, and on the south side 1 in 36. There are four vertical shafts, two on each side of the river, and varying in depth from 75 ft. to 100 ft. below ground level. Each shaft is a wrought-iron caisson of 58 ft. external diameter at the bottom, and 48 ft. internal diameter throughout, and lined with brickwork. Each caisson consists of two wrought-iron skins, 5 ft. apart, braced together, and terminating in a cuttingedge. Two circular holes, which are temporarily plugged while sinking, are left in each caisson to give way for the tunnel through the shaft, and provision is made for an airtight floor above the level of the tunnel when necessary. The space between the two skins is filled with concrete. Two caissons have been sunk, and the two others are in course of being sunk. The tunnel is constructed of cast-iron rings 2 ft. 6 in. long, and each ring consists of fourteen segments and a key-piece. The thickness of metal is 2 in. and each segment has flanges 12 in. deep, and both longitudinal and circumferential joints are planed. The shield used for the construction of the tunnel is 19 ft. 6 in. long, and is 27 ft. 8 in. in external diameter. The outer shell consists of four -in. steel plates. The shield is divided into a front and back portion by two vertical diaphragms at right angles to its axis. It is thus possible, when necessary, to have a higher air pressure in the working face of the shield than in the completed portion of the tunnel. The space between these two diaphragms forms an air-lock, both diaphragms, of course, being provided with doors, by which access to the working face is obtained. At the back of this air-lock the shield consists only of the outer shell, which always laps over and outside at least one completed ring of the tunnel, and inside of which all the rings are built. The space of 4 in. left outside the rings when the shield is shoved forward is filled with grout, forced in by air pressure through screwed holes made in each segment for the purpose. Everything is, therefore, quite solid at the back of the cast-iron lining. At the air-lock and in front of it there is an inner shell, connected stiffly to the outer shell by circular girders and in other ways, and both joining together at the cutting-edge. The working face is divided into four horizontal floors and twelve working chambers by vertical and horizontal diaphragms in the line of the axis of 1894. 3 c |