June 8, 1917. THE COLLIERY GUARDIAN. 1077 and also in works where the processes are continuous and the loss of men has been heavy, e.p., mineral oil, explosive works, and electric generating stations, but as a rule in these cases it has been possible to give the workers another day off instead during the week. Most of the inspectors refer to the improvements made in factories with a view to raising the standard of health and comfort of the workers, especially where women have been introduced. Instances are given of greater cleanliness, better heating, lighting, ventila- tion and sanitary accommodation, improved first- aid and ambulance arrangements, and the provision of protective clothing. In many cases occupiers have provided tea in the afternoons free of charge for those working overtime, with very beneficial results. Whilst the observance of the Orders has been, on the whole, satisfactory as regards the periods of employment allowed, cases have been met with where the limits fixed have been deliberately exceeded, or where excessive overtime has been worked without any notice or application being made. Important prose- cutions have also been taken for failure to fence machinery and plant; in three of these cases, which were taken under section 136, a penalty of £50 was imposed. There has again been a large extension of munition works of all kinds, and important developments have taken place during the year in the manufacture of explosives, chemicals, glass, hosiery needles, and other industries concerned with articles formerly imported. WIGAN MINING AND TECHNICAL COLLEGE. JUNIOR COMMERCIAL SCHOLARSHIPS. Mr. S. C. Laws, M.A., B.Sc., the Principal of the Wigan and District Mining and Technical College, in a communication which he has forwarded to Mr. G. H. Mockler, Director of Education for Wigan, states that after considering a memorandum on the commercial training of boys and girls from the Wigan Education Authority, the governors of the Mining and Technical College have decided to take the necessary steps to establish: — (a) A senior day commercial course of two years’ dura- tion for boys and girls leaving secondary schools at about 16 years of age, provided the enrolment amounts to 12 pupils ; the tuition fee for the course to be £6 per session. (b) A junior day commercial course of two years’ dura- tion for boys and girls leaving the elementary schools at 13 years of age, provided the enrolment amounts to 24 pupils ; the tuition fee for the course to be £3 per session. The governors also invited the Wigan Education Committee to offer a limited number of scholarships to suitable boys and girls desirous of attending the junior commercial course. In making the suggestion as to scholarships, the governors considered carefully the memorandum adopted by the Education Committee, and while firmly supporting the view there expressed that “it is not considered desirable to offer any inducement to boys well qualified to attend the industrial course, who might wish to take instead the commercial course, with a view to wage-earning without the necessity of serving an apprenticeship,” yet, on the other hand, they are strongly of opinion that some provision should be made so that a boy or girl whose natural aptitude appears to favour a commercial rather than an indus- trial career, may not be altogether excluded from the course by reason of the inability of the parent to assume the whole of the financial obligation involved. The Wigan Higher Education Sub-Committee recommended the Borough Education Authority to offer 10 scholarships annually for boys and girls of 13 years of age leaving the elementary schools in the county borough, such scholarships to entitle the holders to free admission for two years to the junior commercial course to be established at the Mining and Technical College, together with a maintenance grant of £5 per annum. These recommendations were adopted by the Educa- tion Authority last week, when Mr. Laws addressed the members on the college scheme. Mr. Laws referred to the proposed junior commer- cial course scholarships at the Mining and Technical College, and pointed out that commercial instruction at the college was at present confined to evening classes. In this section there were about .400 students, a number indicating the need for more adequate instruction than could be provided by means of even- ing classes; and this need was confirmed by the appli- cations which had been received for full-time instruc- tion in commercial subjects. He thought that the number of applications would be largely increased were the distinctions between the full-time course and the part-time course properly appreciated. The even- ing part-time classes met, say, on three evenings a week, having a maximum of 7| hours, and studying 25 weeks in .the year, gave a maximum of 200 hours for the session; whereas in the full-time course, which gave approximately 30 hours weekly, and continued for 36 weeks, students received 1,000 hours of instruc- tion during the session, so that one session’s attend- ance at the full-time day course would be equivalent to five years’ attendance at the part-time evening course. That was a point which was very largely over- looked at the present time. He had recently attended a conference of the heads of secondary schools con- vened by the Director of Education; and the unani- mous feeling of that conference was represented by the memorandum which the Director had prepared for the Education Committee, and which was ultimately adopted. The memorandum suggested that the governors of the Mining College should establish two distinct types of commercial instruction: a senior course intended for pupils who had already had, say, four years’ instruction in a secondary school; and a junior commercial course for pupils who had completed an elementary school course. Most of the students taking up a commercial career would not be able to satisfy the conditions for entering upon the senior commercial course. The college found, as a matter of fact, that two-thirds of the students attending the commercial classes came direct to the college from the elementary schools. The junior commercial course was intended for such pupils as these, who would receive instruction in the technique of the office and the counting house, and in such subjects as short- hand, book-keeping, and typewriting; and there would also be the opportunity of continuing their education along general lines, by providing instruc- tion in subjects like English history, geography, mathematics, and physical science. Perhaps oppor- tunity would also be given for learning a modern language, so that the course would not be narrowly utilitarian, but would provide, to some extent, for the general education of pupils. It was, however, not intended that the course at the college should compete with the normal secondary school course. Parents who were able to maintain children at school until they were 16 years old would, if they were wise, arrange for them to attend a secondary school at 11 years of age, and if, on reaching the age of 15 or 16, a pupil contemplated a commercial career, then the senior commercial course would be open for such pupils. He believed a fairly large number of parents would be willing to give their children on leaving the elementary school a further two years at the college, attending the junior commercial course before sending them out into the world, and he had no doubt that the Education Authority would feel that a course which was going to extend for a couple of years the period of full-time attendance on the part of any considerable portion of the population was one which ought to be thoroughly supported. THE COAL QUESTION IN FRANCE. Although the production of coal in France has increased considerably of late—the figures having risen from 1,800,000 tons in December last to 2,100,000 tons in January, 1,900,000 tons in February, 2,367,000 tons in March, and 2,225,000 tons in April—the diminution of imports has led the Ministerial Council to release from military service the miners called up in the classes 1907-1909, in order to improve the home output still further, and lessen any possible shortage next winter. At the same time, M. Viollette, Minister of Supplies, has introduced a Bill into the Chamber of Deputies to prevent coal hoarding and speculation, both of which evils are stated to be rife at present., The Bill pro- hibits any attempt to deal in coal on the part of any persons who were not in the trade on January 1 last, under pain of one to four months’ imprisonment and a fine of 1,000 to 10,000 fr. No one to whom coal is consigned for a particular purpose may divert it to any other destination, except that certain industrial firms may distribute coal to their permanent staff under existing custom. In a circular addressed to the National Coal Bureau with regard to the importation of foreign coals, the Minister indicates that applicants for licences should give proof that they have boats, and that the licences should be for definite quantities and qualities of coal, to supply needs that have already been found justifi- able. This will stop fictitious licensees, and restrict licences to genuine importers and to authorised trade groups. In fact, no one will be classed as an importer who is not already enrolled as such, or cannot show that he possesses the means of unloading, stocking, and transporting the coal. . At each group of ports the Bureau is to form a group of importers to take charge of the receipt and despatch of coal. Moreover, the ultimate destination of the coal is to be ascer- tained, the principle to adopt being its distribution to the best of the national interest. Coal intended for recognised trade groups, associated municipalities, departmental offices, or other recognised organisations, is to be considered as part of the allotment to the department in which they are domiciled. It is desir- able to fix only the price payable to the importer when the coal is to be diverted; and a system has been devised under which the importer will have a reason- able profit, whilst the possibility of speculation is pre- cluded. Any disputes are to be arbitrated on by the control committee at each port. With regard to the cost of freight on time charters, the recent meeting of importers asked that the Government should take over all charter parties, but leave the management of vessels to the importers; but in order to prevent the withdrawal of vessels, on one pretext or another, under such a system, it has been decided to limit its application to those importers who desire it. The price of coal will be dealt with in a subsequent circular. Briquettes from Ashes or Dusts.—In connection with a paper read before the Society of Architects on June 7, Mr. R. Goulburn Lovell exhibited specimens of coals which had been concreted from coal dust alone, coke dust alone, destructor ash alone, destructor ash and coal dust, and clinker, bar ash, and sawdust. The better grades burned like natural coal, the lower grades like coke; and all flared, coked and incandesced. The nature of the process was not disclosed, but it was stated that the machinery and plant required practically exist in all industrial centres, viz., crushers or breakers, screens, drying tables, mixers and moulds such as old pails, baths, etc. For the cooking process, either ovens or boilers can be used, the main object at the present time being to utilise any existing plant with unskilled labour. The cost of the labour and establish- ment charges would not be more than 5s. a ton. It was claimed that coal slack can be concreted, without pressure, into an excellent coal, and all powderous brown coals into a good transportable coal; that all coke dust accumulating in gas works can be similarly utilised, an excellent fuel produced from sawdust in all the timber countries of the world, and household municipal destructor ashes can be made into cheap flaring coke ; that all industrial concerns consuming more than 10 tons of coal a day can reduce their bill by at least one-third, and also use of their accumu- lating waste products, and that this work can be done by existing machinery and unskilled labour. SUBSIDENCE RESULTING FROM MINING.* By L. A. Young and H. H. Stobk. THEORIES OF SUBSIDENCE. (Continued from page 1044.J Opinions of American Engineers. A number of prominent engineers have made investi- gations as to the nature, extent and cause of the damage to property resulting from surface subsidence in Scranton, Pennsylvania. In discussing subsidence in the longwall district of Illinois, G. S. Rice said the roof settles most in the first few months, but it is several years before it is entirely settled, by which time the gob has been squeezed down to one-half or one-third its original thickness. The roof is very free from slips and vertical cracks or joints until the coal has been mined below it, but when the coal is brought down in a long strip, it marks the roof just where the break of coal has occurred, and along these marks the roof afterwards breaks. The breaks seem to run up indefinitely, and oftentimes they can be followed up to the black slate, 8 or 10 ft. above. As a result of mining the seam, which varies in thickness from 2 ft. 10 in. to 4 ft., or an average of 3 to 3|ft., the settling of the roof is appreciable at the surface even when the seam is at a depth of 400 or 500 ft.; but so gradual is it and without vibration that the deep mines have caused no trouble in going under rail- road tracks, and even under brick buildings, as has been done at La Salle. S. A. Taylor has pointed out the fact that the tendency of one measure to slip on another is counter- acted by the fact that the roof is a continuous mass, so that as soon as any part tends to give, it has to slide to get past adjacent surfaces and the surfaces are pressed together with no little weight. If any part is weak, it is nevertheless held in place and the strain goes to the other measure. In his opinion the roof rock acts as a monolith in most cases. He also believes that the occiiiTence or absence of subsidence depends on the height and character of the overlying strata, and that the rule set down by the English authority “ one to ten ” will not hold, its truth or falsity in any instance depending on the character of the overlying strata. R. D. Hall has suggested that when the roof sags down over the edge of a pillar the curve of the roof tends to follow back over the solid coal, criticising the general notion that the roof lies flat upon pillar and then sags down over the edge of the pillar. In dis- cussing the strength of mine roofs R. D. Hall suggested that the roof over rooms acts after the first fractures not like a beam but like an arch, continuous beams or 7/ " Fig. 13.—Stresses in Arch. plates being replaced by disconnected arches or vaults; and that there is a progressive advance in demolition: first, a condition symbolised by the tunnel in solid rock, in which roof and sides and floor all partake of the beam strain ; second, a horizontal shear which converts the sides into mere supports and the roof into a true beam or plate; third, a rupture of the roof which converts it into an arch, and finally, a failure of the arch or vault by one of the many weaknesses to which such structures are subject. The tendency of the roof to arch has long been noted, and the mechanics of natural rock arches has been dis- cussed by a number of engineers. However, there has been little agreement among engineers as to the portion of the burden of the overlying beds which is actually borne by such natural arches. The strata acting as a uniformly loaded horizontal beam cannot support a great load, and as the strata sink the upper measures tend to arch and eventually the entiie mass maybe supported by the arch. The theory of the arch as applied to this problem has been discussed by B. S. Randolph as follows: In the arch ABC, fig. 13, the two sides AB and B C are mutually supported at B whei e the thrust is horizontal. Assuming the load to be evenly distributed over the arch, it is found that the points B, G, K, J, L all lie in the line of stress. This line of stress when lying in solid material over an excavated cavity will constitute, for all practical purposes, an arch supporting all the material above it and allowing the removal of all the material below it up to the point where this material becomes effective in resisting the stress. There will exist along and on each side this line of stress a zone of material under more or less pressure, depending for its width on the total stress and the elasticity of the material. The position and character of the forces acting on the arch will vitally affect the shape of this line of stress. In an arch under a perfect fluid, where the pressures are all radial acting toward a common centre, the line of stress becomes the arc of a circle. With an excess of load toward the centre, it takes the shape of a parabola, the focal distance shortening as the central load exceeds that on the side. With the excess of pressure on the sides, say at an angle of 45 degs., it assumes the shape of an ellipse, the focal distance shortening as the pressures at the side exceed those in the middle. In the arch formed over rooms, * From University of Illinois Engineering Experiment Station Bulletin No. 91.