244 THE COLLIERY GUARDIAN. January 30, 1914. been completed. The arch shown in fig. 4 is of large proportions, being 18 ft. high, and 25 ft. wide at the floor. The method may be applied in the case of large semi- circular arches in much the same manner, putting in the upper portion of the arch as shown in fig. 5. This being completed the work of excavating proceeds to the floor of the road, leaving a ledge extending almost to the bottom. Transverse supports o may be set at intervals if required to prevent the sides collapsing. The ledges k are now removed in sections and the walling q put in. After the side walls are built to the specified height, a second section of arch would be built on centres resting on the supports p or on planks. In the foregoing remarks it is assumed that as one section of the arch is completed operations are at once commenced to complete that “length” by taking out the ground below. This method would entail much ...I.....• 0 | • o o 1/ Fig. 4. I i 0- 7///AV Fig. 5, ~T i i i Fig. 6. labour in handling materials, though possibly not more than in the ordinary method. This plan, however, need not necessarily be followed. Indeed, the work might be facilitated if the whole length of arch, say 50 yards or so, was made first of all in the upper section before commencing to excavate downwards. The men then return and put in the lower portions. It will be observed, however, that the height from the floor of the first drivage to the underside of the arch is only about 4 feet, but this may be increased by lifting up a portion of the floor, as shown in dotted lines in fig. 2. In this way a line of rails is kept forward as the face advances, the debris is readily handled, and all the necessary materials, such as timber, bricks, mortar, &c., conveyed where required with ease. Also, the arch would be allowed time to settle, and the centres thus removed when the work in the lower sections proceeds. The removal of the needles /, fig. 4, should then be a comparatively easy matter. In any of the cases described there should be no special danger of shot-firing except at points near the arch, as the structure is not likely to be disturbed in ordinary circumstances. Care must be exercised, how- ever, in removing the ledges, especially when approaching the arch, but light shots may be used in the lower portions when required. Figs. 6 and 7 show the application of this principle in the case of an old heading which is to be enlarged from 7 feet by 6 feet for an arch 18 feet wide and 21 feet high, internal dimensions. Fig. 6 illustrates the manner of timbering the first drivage at the top, and the dotted lines indicate the amount of ground to be taken out. Fig. 7 is a section through such an excavation, but shows one “ length ” of arching at the top adjoining a complete “ length,” and the ground being removed from above the old timbers for a further lower “ length ” of arch. If the old road was required to be kept open precautions would be taken to see that the rock was taken out gradually, so as not to interfere with the ventilation. The advantages to be derived by carrying the top section of arch forward the whole distance before working below become apparent from fig. 7. The writer is of opinion that this method of pro- cedure would have the following advantages :— Less timber required owing to the small area that would be temporarily supported; no timber being necessary for the face as in other systems. All the timber is readily recovered and may possibly be used over and over again. Saving of labour in setting timber, because the men are engaged near the roof, and the amount of scaffolding required would also be reduced, thus effecting considerable economy in this respect. Fig. 7. Increased safety, especially in the work of with- drawing timber, and the work’ may be more speedily carried out on account of the increased safety, and the fact that in the main part of the operation there would be little danger of disturbing the supports. A uniform distribution of pressure will be ensured owing to the complete withdrawal of the timber. Ton-Mile Statistics.—A decision of the U.S. Interstate Commerce Commission in the case of the Traffic Bureau of Nashville v. the Louisville and Nashville Railroad Company is of special interest because of the elaborate discussion it affords with reference to the use of statistics showing ton-, car- and train-mile movements as a basis for the making of rates. In this connection, speaking specially of coal move- ments, the Commission says: Ton-mile statistics, reflecting as they do neither car loading, train tonnage, nor car- or train-mileage, are far from being infallible guides in fixing freight rates. A high average ton-mile revenue may be due to short hauls, a preponderance of which occasions the railroad traffic manager much uneasiness, while it has been repeatedly shown that traffic low in ton-mile earnings may, because of its farther carriage and greater density, be the most remunerative. Per-car earnings, with distance con- sidered, are much more reliable. Where the commodity moves in trainloads the earnings per train-mile furnish the best criterion, not only the car loading, but also such physical conditions as grades, &c., being here reflected. Comparisons of any kind, however, to be effective must be analogous, or nearly so; that is, the rate charged or gross earnings derived on any basis for the transportation of a given commodity between two points furnishes a guide in arriving at the rate to be charged upon the same or nearly the same commodity between two other points similarly circumstanced. Comparisons made with coal moving to the lakes for transhipment, to tidewater and between points in central freight association territory are of little value here because of the manifest difference in transportation conditions, particularly with respect to density of traffic, train tonnage, and return empty hauls. MODERR GAS-ERGIM PRACTICE FOR COLLIERIES* By A. T. Cocking. The economical generation of power at collieries is now receiving much more attention than it has ever had, and notwithstanding the fact that fuel must always be cheaper at collieries than at other places, where its cost is increased by carriage and handling, still there is probably no user of power who is likely to benefit to such a great extent by improvements in gas engines and their appliances as the colliery engineer. The possibility of generating gas from waste fuel purposely left in the mine, or picked out as useless in sorting the coal, must now be recognised as a means of supplying most if not all the power required for the work of a colliery, and this source of power, added to that recoverable by utilising the gases generated in coking coal, points to the probability that in the near future a modern colliery will be in a position not only to generate its own power without using its saleable fuel, but will also from the same source supply electrical current to neighbouring factories for power, or to town- ships for light; and, taking into consideration the unique advantages which a colliery possesses in having its saleable fuel at first cost, it is not an extravagant prediction to say that in the future, by means of a judicious blending of the cheaper kinds of its saleable fuel with its available waste, and by the full use of the products of its coking plant, an up-to-date colliery will become a great power-generating centre, the revenue from which will be the predominating factor in its profits. Independently of the great advantage possessed by a colliery in having such an available force in its waste products, the economical conversion of its saleable fuel into power has not received the attention that it deserves. A comparison of the value to be obtained from this fuel in generating steam under the conditions which have usually been adopted at collieries, with the results now obtained by using gas power, will show that even if a colliery had entirely to depend upon the use of its saleable fuel for producing its power, the economical advantage of using it in the latter form is sufficiently important to make it advisable to consider whether it will not justify the replacement of the existing machinery by modern gas power. When the power plant has to be designed for a new colliery, a convenient central power-station can be arranged in a suitable position, leaving room for exten- sions to meet all reasonable requirements, which may at a future time be added with a minimum of expense, and without any interruption of work. An important con- sideration in such a case will be to determine the unit of engine most suitable for the purpose, as regards its size, type and maker. Within reasonable limits too much consideration should not be given to the test efficiency of the engine. It is much more important in colliery work to have the simplest and most reliable engine. Complication in design should be avoided, even if it claims to effect a saving in fuel, the real economy of an engine being determined by the total cost of operating it, and this cost includes upkeep and repairs as well as fuel. It must also be remembered that the loss due to a stoppage may wipe out in an hour or two a year’s saving in fuel obtained in this way. A simple well- built engine usually proves itself to be the most satis- factory machine. This in principle applies to all users of power, but to a much greater extent in colliery work, where the fuel is cheaper, and where uninterrupted and regular running is so important to the working of such a machine as a colliery ventilator. It must be remembered that the efficiency of a gas engine varies greatly with its load. It is therefore preferable not to adopt a unit of engine of too high power, which might only occa- sionally be required to work at its maximum power, and would at other times be working very uneconomically under a reduced load. It is better to have a number of comparatively small engines always working at their rated power, and as the power required varies, start up or shut down units in order to keep each engine working at its full load. The efficiency of a steam engine increases rapidly with its size, but this is not the case with gas engines. Smaller units give almost the same efficiency as large ones, and the maximum economy is probably reached with an engine developing about 300-horse power. It is certain also that great regularity can be obtained by employing small units, as the effect of one or two engines working defectively is hardly noticed in a number of small engines, while such an event Would have serious consequences where the power is generated by a few large engines. For the same reason the cost of providing a sufficient excess of power to allow for repairs is reduced with the size of the unit, as one or two engines may be taken off for repairs without greatly reducing the available quantity of power, but the effect of taking off a large engine, repre- senting a great proportion of the total power, would be serious. * From a paper read before the Midland Institute o Mining, Civil and Mechanical Engineers.