precautions ought to be taken. No doubt the tunnels, for instance, Mr. Ross. ought to have inverts, and the foundations ought to be perhaps better than usual. As for the viaducts, he did not suppose arches could be dispensed with. He was not so afraid of arches as had been anticipated; but there was no doubt that in single bridges a metallic superstructure had great advantages. It had been his practice for many years to divide those superstructures so that each line of rails was self-contained. The great advantage of that was that when subsidence had taken place and the railway had to be lifted, one line could be lifted at a time, and the other could remain in use. With regard to arches, in many cases of his own he had to pull down an arch, especially an arch over a railway. If there was only limited headway to start with, and there was subsidence at all, and the line was kept up, the arch came within the gauge and the rolling-stock would come into contact with it. Generally for that reason alone overlying arches had to be pulled down when there was serious subsidence. But, after all, it was largely a matter of degree. The case that the Author had referred to, was that of a subsidence caused by one seam only, but in most of the coal-fields -and in some more than others-settlements had scarcely taken place from the subsidence caused by one seam, when other seams were taken out, so that the subsidence due to coal working might extend not only over 5 years, but even over 20 years; and he had known in his own experience many cases of that kind in which the extent of subsidence had been very great. In the case of buildings, the precautions necessary and obvious were to provide framed buildings at the stations, when the lifting would be much easier, and the damage would be much less. As was well known, an existing railway company, before the working commenced, received due notice of the intended coal-getting under the Clauses Act, 1845, and they had 30 days to consider what was the best course. The mining engineer usually made his report to the engineer responsible for the safety of the works, and he decided whether to recommend purchase or not. He was not much in favour of purchasing unless he was actually compelled to do so. There were cases where, for the safety of the line, it was obvious that coal purchases must be made, and in the case of several seams of coal lying underneath tunnels and viaducts, sooner or later purchase would become necessary. He had found, from many years' experience, that the mining engineers' reports told with very great accuracy what was going to occur; but with coal-mining, although not quite so much perhaps as with other

Mr. Ross. mining, the unexpected did happen. There was the pull and the draw, and in many cases the pumps working in a wet mine drew quicksand for long distances, and the effects were felt at such long distances that it was even doubtful which mine was causing it. Therefore he thought it was obvious that a definite procedure could not be stated that would help in deciding, under all circumstances, when not to purchase the coal. The whole disturbance has to be carefully watched and dealt with according to the effects.

Mr. Worthington.

Mr. W. B. WORTHINGTON desired to add a few remarks as to the relative qualities of the arch and the girder bridge. He did not go quite so far as Mr. MacDonald, but inclined towards the view expressed by the Author. He had never had to take down a girder bridge from this cause, yet he had had to take down several arches, owing to their failing through coal subsidence. With regard to the point named by Mr. Ross, namely, the superiority of the girder bridge to the arched bridge over a railway, if 5 feet of coal was worked underneath a girder bridge, and it subsided 3 feet 4 inches or so, the girders could be lifted easily without interfering with the traffic on the railway, but if the bridge over the railway was a stone or brick arch, unless it was abnormally high to start with, it was necessary either to let the railway stay at the level to which it had subsided or pull the arch down. That was a very important point with lines which were so crowded with traffic as the railways which passed through coal districts. With regard to the pillar to be left for the support of structures, he had had an opportunity of observing the effect of leaving pillars; and he thought there was no doubt that greater damage was caused to the structure on the surface from leaving a pillar which was too small than from the entire taking out of the coal. If the pillar was too small the pull at the structure on the surface from all sides was such that it was very seriously damaged. With regard to the gridiron pillars it happened that within the last few months he had met with a case which threw a little light upon that subject. In a railway which was made 48 years ago the line was made above a bed of coal which had been worked on the pillar-and-stall system, rather more than half the coal having been got, and the spaces were partly filled in with rubbish. The seam was near the surface-in fact, running out of the surface at one point and gradually getting deeper and deeper. At a point where it was about 5 yards deep a subsidence had occurred within the last few months, the top crowning in. That appeared to show that the pillars and roof had decrepitated

during the 48 years. They had carried the weight up to the end Mr. Worthof 48 years. ington.

Mr. W. WALTER ROWLEY exhibited upon the screen four photo- Mr. Rowley. graphs illustrating the effect of subsidence of about 4 feet upon the banks of a canal, under which two seams of coal amounting to 7 feet 6 inches in thickness had been extracted, at a depth of about 200 yards. When he had first, many years ago, to consider and apply experience previously gained in mining operations to the working of minerals under, and the support of, railways, canals, reservoirs and other engineering works, he thought the task would be easy if the rules governing the size of a pillar to support a building could be reduced to a formula. But the longer his experience the more he was impressed with the difficulty of arriving at such a satisfactory issue. He found in practice the laws of theory could be used only to a certain extent in framing a general rule, which had to be modified according to the circumstances and surroundings of each case. According to a rule sometimes recognised, the pillars should extend on all sides to a distance equal to one-tenth of the depth of the seam plus 20 yards. Such a case as that given by the Author, namely, 400 yards deep, would give a fringe of 60 yards. This rule is applicable, as an approximate basis, to horizontal mines in which the thickness of the seam did not exceed 6 feet. In theory the circular pillar was correct, but in practice it was not found so desirable; often a square, rectangular, or still more irregular shape, had to be adopted in order to meet the special necessities of the case. One of the first points to be considered was the amount of subsidence that could be expected, because if that was small, the question of support might possibly be dispensed with. Such a question was affected by packing and other considerations. In considering the extent of support, the compensation to be paid to the mine-owner was often considered a very serious item to deal with, but he had sometimes found it difficult when advising that no pillar be purchased, to justify throwing on the engineering department such a grave responsibility as rested with them in maintaining the permanent way. From his record of over six hundred cases he had dealt with he found that less than 10 per cent. had had pillars purchased for their support. Protection for the railway by a pillar was never sought for unless the gravest sense of economy, safety and convenience, rendered it necessary. In working coal under bridges the method to be employed had to be taken very carefully into consideration. The advantage of the face of coal workings approaching a bridge broadside was shown in Fig. 7. If coal could be worked rapidly in this

Mr. Rowley. manner the minimum amount of damage would take place; but if the bridge was approached end-ways, the girders were found to

Fig. 7.

yield and the buttresses gave way; and if there was any stoppage in the operations of the mine, the effect upon the bridge was far more serious. In Fig. 8 was illustrated the first principle of support, namely, as far as possible to utilize the existing workings of a mine when worked on the pillar-and-stall system, so as to provide, with little interference to the mine

LONG-WALL FACE APPROACHING A BRIDGE which meant the minimum

BROADSIDE.

of cost to the railway-an adequate support. An adaptation of the existing blocks or pillars of coal was also illustrated in Fig. 9, dispensing to a great extent

Fig. 8.

with the important question of severance by simply recognizing the existing pieces of coal, and ordering them to be left in such a position as would best serve and protect the property. It had been mentioned by Mr. Elliot Cooper that in a line in which he was concerned no pillar had been purchased for the protection of the works, except in one case. That line ran from west to east, beginning at Chesterfield and extending to Lincoln. A few miles distant from Chesterfield it left the exposed coal measures, which had an eastwardly dip under the Permian rocks and New Red Sandstone, and gradually SUPPORT FOR A BRIDGE IN PILLAR assumed a great depth, rendering the difficulties of subsidence

AND STALL.

comparatively slight. The coal-field was not yet opened up under the greater portion of the line, and in the only case where

there was any necessity for the serious consideration of the effects Mr. Rowley. of subsidence, namely, an important viaduct, support was purchased. The case of this railway was very different to the old mineral lines which had a very great mileage in the heart of the exposed coal-fields, with minerals varying between 2 feet and 30 feet in thickness, being worked at all depths. Modern railways had the advantage of experience, obtained at great cost by the older systems with regard to the difficulties of subsidence, and in some cases circumstances might enable them to so construct the works as to allow of a moderate subsidence. In his experience of the older lines which were made before the extending character of the coal-fields was properly understood, there had been such a limited margin in the height of some bridges that the rails had had to be lowered-a process which could only be carried on to a very limited extent, and in the end sometimes requiring the bridge to be rebuilt. He would only add that in judging of such an important matter it was desir

able not to base conclusions on excep

Fig. 9.

OF WORKING.

tional cases, for proper study of such a subject could only be carefully undertaken from the consideration of many cases distributed over a wide area of observation.

Mr. R. J. GIFFORD READ remarked that the subject of the Paper Mr. Read. had come under his notice in the case of bridges he had been concerned with in South Wales. A short time ago he had been employed by the Council of the Rhondda Valley to design the steel-work of some bridges that they were reconstructing over the River Rhondda. He had laid on the table two photographs of the old stone-arch bridges, which had been taken down. One of those bridges was a single-arch and the other a double-arch bridge-the arches being built of brickwork and the outsides faced with stone. Rhondda Valley was well known for its coal-workings, and the bridges were in the very middle of the district where the coal strike had occurred a short time ago. The bridges had been built 30 or 40 years, but settlements were first noticed about 8 or 9 years ago, and then they