Mr. Essor. The machines would have been about the same weight, but the fall of potential between no load and full load would have been much less; and this was the direction in which improvement was sought, the primary object being to get a small fall of potential rather than diminished weight. He agreed that a system should always be chosen which, in view of all the surrounding conditions, was the simplest and most efficient, involved the least capital outlay, was the most competent to withstand overload, was the least costly to maintain and the least likely to cause delay through breakdown, and it was consequent on weighing up all these circumstances that the 2-phase transmission, as described in the Paper, had been adopted for the Sheba Mine. As regards the amount of overload which would draw up the motors, very great improvements had been effected in the construction of induction motors during the last 4 years, and they were now made so that they would withstand the larger sizes 50 per cent. overload, and the smaller sizes 100 per cent. overload. He had advised the employment of no particular motor under all circumstances. Both induction- and direct-current motors had their uses, and the former should not be altogether condemned because Mr. Slater Lewis' experience had been confined to the latter. The 3-phase installation at Moodie's Mine, and other installations of the same class near Johannesburg, were all subsequent to the installation at Sheba, which was the first on the alternating-current polyphase system in South Africa. The extremely unsatisfactory working of the first power installation at the Moodie Mine, on the directcurrent system, was not without influence in determining the adoption of alternating current for the neighbouring Sheba Mine. The relative value of two phases and three phases, he would reply, was not determined wholly by the saving of copper in the line; many other considerations had to be taken into account, and neither could, under all circumstances, be considered preferable. The President. 22 November, 1898. WILLIAM HENRY PREECE, C.B., F.R.S., President, The PRESIDENT said he had no doubt that every member present had seen the announcement of the death of the oldest and senior Past-President of the Institution. He need not tell the members that it had been received by the Council with great grief. They had accordingly passed the following resolution :-"That the The President. Council have learnt with deep regret of the death of their distinguished Past-President, Sir John Fowler, Bart., whose interest in the welfare of the Institution had been maintained during 54 years of membership, he being at the time of his death the Senior PastPresident, and desire to express to Lady Fowler and the members of the family their sincere sympathy in the loss they have sustained." That resolution would be forwarded in due course to the family, and would be taken as an indication of the deep sympathy which the whole Institution felt in their sad bereavement. The discussion upon the Paper "The Electrical Transmission of Power in Mining," by Mr. W. B. Esson, was continued and concluded. [THE INST. C.E. VOL. CXXXV.] 29 November, 1898. JAMES MANSERGH, Vice-President, in the Chair. (Paper No. 3106.) "The Effect of Subsidence due to Coal-Workings upon Bridges and other Structures." By STANLEY ROBERT KAY, Assoc. M. Inst. C.E. THE coalfields [of England form centres of great industries, equipped with all the means of rapid intercommunication and transit that engineers can devise; so that lofty buildings, heavy bridges, or costly tunnels have often to be constructed in situations where workable coal-seams are known to exist, and, it may be, have actually been worked or are being worked. It is therefore necessary, before designing works in such situations, (1) to know the principles of subsidence following the working of the coal, as a guide to the position and character of the works; (2) to have approximate knowledge of the area of coal necessary to be left unworked to protect the structure about to be designed, supposing the coal not yet worked; and (3) to suit the design to the supposition that the coal may afterwards be worked out without any solid pillar being left for the support of the surface. The general effects upon the surface of removing a seam of coal, of varying thickness and depth, will be first considered, both with regard to the actual subsidence resulting therefrom and the time required for its operation. It is unnecessary to point out that subsidence always follows coal-working. It is evident that, the support being removed, the superincumbent mass must sink, to a greater or less extent, dependent upon the thickness of material excavated. Certain geological factors qualify this axiom, such as beds of rock of greater or less thickness, faults &c.; but generally the subsidence is proportional to the thickness of material excavated. The depth below the surface approximately regulates the duration of the movement. Those geological con siderations have at times an important bearing upon both the amount and the manner of subsidence. Where thick and massive beds of sandstone occur between the coal and the surface, as, for instance, the Pennant rocks of the South Wales and Bristol coalfields, the subsidence is very gradual, and, in the case of a thin seam, almost imperceptible, though in that of a thick seam it is sometimes severe and eccentric. .....200 yds Fig. 1. When, during the working of coal, it is found necessary to remove a portion of the overlying shale or clod, this rubbish is used for stowing and wall-packing the excavated area, to avoid the expense of bringing it out of the pit; in some cases material is taken underground from the surface in order that the wastes may be packed particularly tight over a special area; where this is done to a large extent, it follows that subsidence is considerably modified, and the damage to the surface in the case of shallow mines is considerably diminished. It is not, however, practicable in all cases to ' pack the wastes very tight, and the removal of the coal in shallow mines down to, say, 50 yards in depth, causes the overlying strata to give way occasionally quite up to the surface; cracks are formed traversing lines of weakness in the strata and wrecking any rigid structure in their course. SOUD FAULT COAL 350 Yards COAL WORKED OUT Even in the case of deeper mines, where subsidence seldom causes fracture of the surface, the lines of faults are lines of weakness; and where the coal has been worked off to a fault underground, it not infrequently happens that damage occurs to structures upon the surface-line of such a fault, in some cases a considerable distance horizontally from the line of fault in the coal, owing to its hade or inclination. The lateral support of the strata loses its continuity at the fault, and though vertically below there may be apparently a sufficient area of solid coal, it has no effect in preventing subsidence upon the other side of the fault. An example from actual practice, Fig. 1, illustrates the danger to be apprehended from coal-workings near a line of fault. The working of the coal on the lower side of the fault at a depth of 350 yards (though only 3 feet in thickness) induced subsidence which extended to the line at which the fault was found at the surface, 200 yards horizontally from the point vertically over the nearest coal-workings at the fault. Unfortunately a valuable house stood upon the line of fault, and it was greatly damaged. Lines of fault at the surface are therefore to be avoided in the erection of permanent works even if pillars of coal are purchased for support, as it is impossible to ensure against a possible "drag" or "pull over" of the strata, unless an abnormally large area of support is secured; and this upon the score of economy is inadvisable. In the case of an area free from faults, and where the strata are horizontal, or nearly so, the subsidence following the working of coal is a simple mechanical problem, in which few disturbing factors have effect. There is a direct vertical subsidence varying in amount between one-half and two-thirds of the thickness of material excavated-generally the former, but in cases where material for "packing" purposes is scarce and the goaf extensive, the latter ratio is reached. The subsidence from the working of shallow mines down to, say, 100 yards in depth, is felt at the surface within a period varying between a few weeks and a few months, according to the depth and thickness of the seam and the character of the overlying strata. Breaks in the strata seldom find their way to the surface from a depth of 100 yards unless the thickness of the seam worked is considerable and there is a thick bed of rock intervening. The Author is aware of instances of coal-seams up to 5 feet in thickness being worked out beneath canals and rivers at this depth, without the slightest percolation resulting; care having been taken in the packing and stowing of the goaf to reduce the amount of subsidence as much as possible. The time taken for complete subsidence at this depth may be between 2 years and 3 years, varying as already stated. In the case of mines more than 100 yards in depth, the subsidence follows more slowly the greater the depth and the less the thickness excavated, as might be expected. The Author some years ago took levels extending over a period of 5 years on the surface of a portion of two separate colliery royalties, at depths of 120 yards and 330 yards, with a view to obtain reliable evidence as to the commencement, duration, character, and amount of the subsidence. The locality chosen in each case was fairly level, the strata, in addition to being nearly horizontal, being free from faults of any magnitude; and was of the average coal-measure character of alternating binds |