728 THE COLLIERY GUARDIAN. October 8, 1915. CURRENT SCIENCE The Trend of Electrical Development. In his presidential address delivered before the American Institute of Electrical Engineers, Mr. P. M. Lincoln discussed the possible directions of future pro- gress. Dealing first with efficiency, .he showed that there is little scope for further improvement. The con- version of mechanical into electrical energy already reaches in some cases 97 per cent., or even more, and the average loss on conversion scarcely leaves a margin of 10 per cent. Owing, however, to the larger size of the average electrical generator and its greater load possi- bility as compared with the average motor, the efficiency of the conversion of mechanical energy into electrical energy is higher than the re-conversion of the latter into mechanical energy. As regards transformers, the efficiency of the larger types may exceed 99 per cent., and the rotary converter, in which alternating current is changed into direct, attains an efficiency of nearly 98 per cent. Clearly, therefore, there is little room for much improvement here. In the matter of prime movers, the modern water wheel has no limit in head, except in the strength of available materials, and although improve- ments are possible in design, we cannot expect to increase efficiencies more than 10 per cent, in the case of the best designs now available. The turbine has already greatly increased the efficiency of thermo- dynamic conversion, and the best modern steam turbines are now capable of yielding 75 per cent, or more of the “ Rankine cycle efficiency.” This figure, however, is only possible with turbines of large capacity—a fact show- ing that central station supply must always be more economical than isolated plant supply. Since, however, the efficiency of thermo-dynamic conversion depends upon the available temperature range of the working fluid, it may be possible to increase this range, but in the case of steam, neither by superheating, nor by increase of pressure does there seem to be much pro- mise of marked improvement upon existing practice. It has recently been proposed to use mercury as the work- ing fluid, by which means the temperature range can be considerably increased, but the possibilities of a mercury boiler have not yet been liberated from lhe practical difficulties involved. The size and capacity of generating units, already reaching 30,000 kw., have not yet reached their limit. Thirty years ago the Edison steam dynamo of 100 kw. capacity was called a ” jumbo.” The capacity of 4,600 kw. has now been reached, and even larger machines will be made in future, the only limits being those imposed by plant capacity and economic considera- tions. In the meantime, the cost has decreased from 20c. to 2c. per watt of output, and modern prices have even been reduced below 4c. per watt. Design has now approached very close to the physical limits of avail- able materials, and although modern transformer steels are vastly better in regard to hysteresis and eddy current losses, these improvements have been effected at the cost of a loss of permeability. Copper seems destined to remain indispensable for conductivity purposes, aluminium being only possible as a substitute when, as in transmission lines, volume is not an essential element. A more promising line of development in respect of cost reduction is the tendency towards higher operating temperatures, which have hitherto been fixed by the dis- integrating point of fibrous insulation. If this difficulty is overcome by improved types of insulation, the ” burn- ing out ” point of an apparatus will only depend upon the maximum heating capability of copper conductors, which affords a wide range of operating temperature. It appears, however, that any revolutionary improvements must come in a fundamental change of method rather than in the improvement of existing methods. Remark- able progress has been made in power transmission, and maximum transmission voltages have now reached 150,000 volts, distances having increased to 245 miles, but the practical limit has been probably nearly reached in this direction; and the economic limit is near at hand. On the whole, it is to be inferred that electricity has passed from infancy to the adult stage, and the rapidity of its growth must henceforth be restricted in accordance with natural law. Subsidences in Colliery Districts. Dr. A. Eckardt recently published a paper in Gluckauf in which he reviewed the various theories respecting surface subsidences in colliery districts. The well-known tangent law of Thiriart is first discussed. This theory is based upon the principle that the superincumbent strata behave as prismatic masses falling by their own weight, as was originally maintained by Bauneaux. The theory is examined mathematically. If the strata are inclined at an angle a, it is maintained by this theory that the fracture will follow the direction making with the normal to the plane of the strata an angle such that tan tan a The angle of fracture would thus be independent of the nature of the rock, but it is here held that the theory fails to take into account the conditions under which the prismatic column is held by the surrounding rocks. The reactions thus set up introduce entirely new con- ditions. Goldreich, from observations of subsidences in the region of Ostrau, concludes that the hard and brittle rocks of the coal measure strata behave differently from the plastic beds of tertiary age. He concludes that in the case of colliery workings covered directly by tertiary strata the volume of the subsided area corre- sponds approximately to the void produced by the workings. He shows that in the hard stiata below the direction of fracture follows the law of Thiriart, but the soft strata above fall vertically over the area of the workings, accompanied by a lateral drag over a larger AND TECHNOLOGY. area determined by the emergence of the normal to the plane of the strata. Where, however, the tertiary cover is thick, other considerations obtain, and Dr. Eckardt. maintains that in general the surface sinks by a simple bending of the superincumbent beds. He advances, in proof of this contention, the observed fact that at Zwickau the level of the Lake of Swaus was lowered in level without any penetration of water into the workings below. He accepts the views of Goldreich as to the influence of the area of the workings upon the depth of settlement, but he denies that the rate of excavation of the coal seam has any influence upon the depth. Hausse has advanced another view. He maintains that after removal of the coal, the overlying beds fall into the void, and by breaking up at last fill the empty space. The overlying strata then settle by simple flexure under the influence of gravity, the pressure serv- ing to compress the broken masses in the area which has caved in. He gives a construction for finding the limits of the caved-in area, which also defines the limits of the surface subsidence by flexure. Although the pro- cedure is sufficiently simple, it fails, in Dr. Eckardt’s opinion, to accord with the results of experience. The author then proceeds to explain his own views upon the mode of propagation of subsidences to the surface. He confirms in part the theory of Hausse outlined above, and shows that the amount of surface subsidence is con- trolled by the thickness of the worked seam, and by the- system of working, viz., whether packing is adopted or not. The nature of the rock is also a factor. If the- rock above the seam is hard, the caved-in mass/con- sisting of broken fragments, will offer great resistance to compression, and surface settlement will be correspondingly slow. Shales behave otherwise. Conse- quently a sandstone roof permits of working without packing, while a shale roof demands complete or partial packing. The plasticity of the superincumbent strata determines whether the subsidence will be by bending or breaking, and he thus explains why the tertiary cover of the Ostrau region adapts itself to the deformation of the lower beds more regularly than elsewhere. Panel workings if too wide apart may cause difficulties by forming at the surface separate subsidences of an irregular character. It is also well to avoid any changes in the mode of working which would alter the normal lines of subsidence and produce secondary tensions and dislocations. On the whole, it is concluded that the problem .is too complex bo enable general laws to be established. Effects of Damp Heat. ■ The admitted adverse influence of damp heat upon the efficiency of the miner gives interest to a paper by Surgeon D. A. Mitchell, R.N., published in the Journal of State Medicine, upon the effects of damp heat observed by the author -in the Persian Gulf and else- where. By damp heat is here meant a dry bulb reading between 90 degs. and 100 degs. Fahr., and a wet bulb temperature within 5 degs. Fahr, of this. This repre- sents the average condition in summer in the Persian Gulf on board ship, the shore climate, although hotter,, being drier. The nature of the ailments traceable to this damp heat included boils, inflammation of the external auditory meatus, ulcers, prolonged low pyrexia, anaemia, pelagic oedema, or beri-beri, slow healing of wounds. The author states that a consideration of the cases under his notice, and their behaviour under the different climatic conditions, has led him to the conclusion that “damp heat,” of itself, frequently produces an altera- tion in the blood, visible only as anaemia, and that this alteration is primarily responsible for the difficulties encountered in the treatment of other conditions. If this -alteration in the blood could be thoroughly investi- gated by elaborate chemical and bacteriological methods, a remedy, probably preventive in nature, might be found for the large amount of disease which at present proves so intractable in this climate. It would be interesting to know if similar deductions might not be permissible in the case of ailments attributable to damp heat under- ground. Ventilation of Deep Mines. A paper by Mr. G. Chalmers, superintendent of Morro Velho Mine, Brazil, probably the deepest mine in the world, appears in the Canadian Mining Journal, August 1915. This mine belongs to the St. John Del Rey Mining Company Limited, and reaches a depth of 5,826 ft. The air is hot and damp, becoming rapidly saturated, and a wet bulb temperature of 86 degs. is commonly experienced. The best working conditions are only possible by leading a large volume of air through the deepest workings, and passing it directly to the upcast without allowing it to circulate to the stopes above. The ventilation of this mine has been pro- gressively improved as the depth of the workings increased. In the first place, natural ventilation was sufficient; later, furnaces had to be adopted during the hot season to assist the natural ventilation. As the mine became still deeper a Capell fan was installed at the top of the upcast “ C ” shaft to ensure a more con- stant volume of air. Finally, a Sirocco fan of larger capacity was installed in place of the former, the jutput of which could be increased by additional power, when required. Besides this, auxiliary fans are used. The temperature gradient in this mine is governed mainly by the progressive increase in rock temperature with depth, which has been ascertained to be approximately 1 deg. Fahr, for every 125 ft. But, in addition to this, there is an increase due to the greater density of the air at increasing depths. This has been investigated, and has been ascertained to be under normal conditions 1 deg. Fahr, for every 180 ft. To these causes must be added the heat due to friction of the air current, heat from machinery, lights, decomposition of mineral matter and the bodies of men and animals. The influence of rock temperature can be diminished by cooling and dry- ing the ventilating current. A cooling plant has accord- ingly been installed on the surface at the top of the downcast shaft. For working this about 400-horse power is required, in addition to 200-horse power for driving the ventilation current. It is believed that these improvements will enable the workings to be continued to a vertical depth of 7,626 ft., or even more, even although the rock temperature at that horizon would be at least 126*5 degs. Fahr. The results hitherto have been satisfactory, the efficiency of the miners being unimpaired, although it must be admitted that men accustomed to the climate of Brazil would probably be less affected than those used to the conditions found in England. THE GERMAN AND AUSTRIAN COAL AND IRON TRADES. We give below further extracts from German periodicals that have reached us, showing the course of the coal and iron trades in Germany and Austria :— German Production of Medium Steel in July. According to the report of the Association of German Iron and Steel Manufacturers, the output of medium steel in the Customs Union during July amounted to- 1,138,478 tons, as compared with 1,080,786 tons in June. Of this total, 563,692 tons (542,967 tons) were basic steel, 13,285 tons (13,635 tons) Bessemer steel,. 455,696 tons (428,170 tons) basic open hearth, 23,473 tons (22,819 tons) acid open hearth, 45,771 tons (39,294 tons) basic cast steel, 15,960 tons (15,563 tons) add cast steel, 8,556 tons (8,366 tons) crucible steel, and 12,046 tons (9,972 tons) electro-steel. Rhenish West- phalia produced 657,146 tons (631,756 tons), Silesia 99,690 tons (88,045 tons), Siegerland and Hesse-Nassau 23,877 tons (23,877 tons), North, East, and Mid-Germany 49,419 tons (47,094 tons), Saxony 27,084 tons (19,870 tons), South Germany 5,878 tons (10,942 tons), Saar and Bavarian Rhinepfalz 91,866 tons (81,988 tons), Elsass- Lothringen 98,297 tons (96,838 tons), and Luxemburg 86,221 tons (80,376 tons). German Steel Union Report. The report presented to the meeting on 23rd ult. states that in semi-manufactured products the home trade and that with neutral countries remained at about the level of the previous month. Sales for the current quarter are to be conducted -without any change in price or con- ditions. In railway superstructural material, the requirements of the Bavarian State Railways are higher than last year, but those of the Prussian and Realm Railways considerably lower. Several large contracts have recently been booked for neutral markets, and others are under negotiation. Orders for tram rails are scarce, but the sale of pit rails continues on the old scale. In sections there is no change in the home market, owing to the continued stagnation of the build- ing trade; but there has been a fair demand for structural materials and wagon building. Prices and conditions for sales during the present quarter are left unaltered. German Pig Iron Union. Following the example of the Steel Union, it has been decided that no change in prices shall be made for sales in the present quarter. As compared with the corre- sponding period of last year, rates have advanced con- siderably all round, foundry pig I. being now 94 mk. (79*50 mk.), No. III., 89 mk. (74*50 mk.), and haema- tite pig, 115 mk. (93 mk.), for Rhenish-Westphalia, ex Oberhausen, and other local centres, freights being charged extra for delivery further afield. Ingots cost 102*50 mk. (90 mk.), blooms 107*50 mk. (95 mk.), billets 115 mk. (102*50 mk.), and plate bars 117*50 mk. (105 mk.). In these products the prices for Sept.-Dec. 1914 were 7*50 mk. higher than in the previous quarter. Bars are now selling at 140 mk. (110 mk.), heavy plate at 150 mk. (112 to 115 mk.), and rolled wire at 140 mk. (112*50 mk.). These last products, however, are only controlled by “ free conventions,” and whilst prices have risen to a greater extent than the products for which “ cartels ” are in force, there has latterly been a tendency on the part of certain works to cut prices occasionally. Coal Syndicate Report for August. In addition to the particulars given in our last issue, the report states that the pits having a sales agreement with the Syndicate produced, during August, 454,062 tons (261,304 tons) of coal, and had a total distribution of 420,598 tons (159,242 tons), of which 188,078 tons (63,282 tons) were for Syndicate account. The total dis- tribution of coke was 117,170 tons (33,841 tons), 75,924 tons (16,360 tons) being for Syndicate account. During the first 13 months of war, the total coal raised by the Syndicated collieries amounted to 76,062,200 tons (103,496,789 tons), a deficit of 27,434,589 tons, or 26*51 per cent.; calculated distribution, 59,410,814 tons (79,600,843 tons), a deficit of 20,190,029 tons, or 25*36 per cent.; total distribution, 76,868,006 tons (101,515,869 tons), a deficit of 24,647 tons, or 24*28 per cent. New Coal Trading Company. A new coal trading company has been registered in Gelsenkirchen, under the title Handelsgesellschaft des Steinkohlenbergwerks Graf Bismarck, G.m.b.H. This concern is an outcome of the agreement previously entered into beween the Graf Bismarck and Friedrich der Grosse collieries and Adolf Harloff, G.m.b.H., of Cassel, for the formation of a trading company in the event of the non-renewal of the Westphalian Coal Syndicate. The Ironite Company Limited inform us that owing to the rapid extension of their business they find their present offices are quite inadequate, and consequently are removing to 11, Old Queen-street, Westminster.