Mr. Kay. subsidence, and might or might not coincide with the lines of Fayol. This line in mines inclined up to, say, 30° from the horizontal, was modified under the action of gravity to that of a resultant lying between the normal and the vertical, as stated previously. He thought Mr. Longden had failed to notice that the omission he took exception to was given in the example accompanying Figs. 4, viz., that the pillar must be left "all round the structure," or, in other words, from all outside points of the building to be protected. Thus, supposing the bridge had long wings, it might be considered expedient to protect the abutments only and take the risk with the wings, in which case the pillar would be laid out 68 yards all round from the outside corners of the abutments. He failed to see on what grounds Mr. Longden considered his formula inadequate for thick seams near the surface, and inaccurate when the seams were thin and the depth great. He had calculated the line of break that the formula would allow for in the case of a 30-foot seam worked at a depth of 50 yards, and found this to be at an angle of 46° from the horizontal, and surely this was a sufficient angle of repose under normal conditions. Then for the thin seam, assuming it to be 2 feet thick and 600 yards deep, the formula gave a pillar of 3 chains radius; and that from the Author's experience would be somewhere near the mark should it be necessary to leave a pillar for so thin a seam at that depth, and amply sufficient, assuming that Mr. Longden's view, that subsidence became so diminishing a quantity at great depths, was correct. He further did not state, in reference to locks of canals, that it was advisable to leave pillars under them in all cases, but that it was usual to do so; for unless the lock-gates and side walls were constructed in the first place abnormally high, it was necessary either to leave a pillar to maintain the height of the works in connection with the fixed water-level at those points, or rebuild the lock, and this was more a question of comparative cost, as a rule, than of damage. The pack walls, as built at most collieries, were of little use in preventing subsidence, as they were mostly composed of the binds and shales obtained from the falls of roof consequent upon the extraction of the coal, and even when well built had not sufficient strength to permanently resist the crushing force of the superincumbent strata. If, however, they were built of strong stone or other material, which might have to be brought from the surface, the result was more satisfactory and the subsidence greatly modified; although where a succession of seams was worked it could not be sufficiently modified to avoid,

in the case of canals, the risk of letting down the locks below water- Mr. Kay. level, unless specially designed in the first place to allow for subsidence. That there might not be so much subsidence from deep as from shallow mines was very likely, and at 1,000 yards it might be reduced to less than, say, one-half the thickness excavated; but as these great depths were not within the region of practical politics at present, the proportion of two-thirds might be taken as an average factor to work to in designing works where subsidence might be expected. Mr. Shelford's method of building the wings and abutments separate, or self-contained, was to be commended in the case of a large bridge with spreading wings, as the damage to bridges from subsidence was frequently at the junction of wings and abutment. If the pilaster could be built to successfully cover the gaping opening that might be left at the line of division, it was a method that commended itself to the Author, though for ordinary small bridges he presumed Mr. Shelford would not adopt it. That thick alluvial or drift deposits might extend over the area in which damage from subsidence might be looked for was undoubtedly true, and the occurrence of such conditions rendered it extremely hypothetical to forecast the line that the break or pull might extend to at the surface. He agreed with Mr. Sopwith that this phase of the subject was of importance, though the eccentric behaviour of the overlying drift, or newer beds, was difficult to follow, according to any law of subsidence, because breaks sometimes spread far beyond any natural angle of repose, as instanced by Mr. Forster Brown.

6 December, 1898.

WILLIAM HENRY PREECE, C.B., F.R.S., President,
in the Chair.

It was announced that the Associate Members hereunder mentioned had been transferred to the class of

LIONEL FORTESCUE WELLS, B.Sc. (Vic-| JOHN STANLEY WHITAKER, B.Sc. (Vic

The Candidates balloted for and duly elected were: as