January 21, 1916. THE COLLIERY GUARDIAN. 119 sity for a shaft at the coupling end, and the coupling can be cast in one piece with the rotor frame, the connection being made of sufficient rigidity to resist torsional oscillations. Air Mains. The author would like to draw attention to a useful diagram for solving problems in connection with air mains. Formulae for calculating .air pipe sizes are some- what complicated and tedious, and it is often necessary to arrive at the result by a series of approximations or trial solutions. In order to facilitate the solution of such problems, Prof. C. R. Richards, of the University of Illinois,* has evolved a transmission diagram for air by which problems pertaining to the flow of air in pipes may be solved quickly and with sufficient accuracy for ordinary requirements. The diagram is based on the following formula :— V = 3-061 . ~ . V KL where V — the number of cubic feet of air at 70 degs. Fahr, and 14’7 lb. absolute pressure flowing per minute; d — the actual diameter of pipe, in inches; — —2 — ratio of t he final to the initial pressures; K = the coefficient of resistance ; L = the length of the pipe, in feet; and Pi = the initial pressuie in the pipe, in pounds per square inch absolute. The use of this formula is simple. If the volume of free air per minute, the length of pipe, the initial pres- sure, and the permissible drop in pressure are stated, the procedure is as follows :—From the length of pipe, in feet, follow a vertical line to its intersection with the volume line; from this point follow a horizontal line to its intersection with the initial pressure line, and thence follow a vertical line until it intersects 'the horizontal line through the given pressure drop. The position of this last point determines the proper pipe diameter. Any other combination of given data is handled similarly. For example, let the length of the pipe be 2,000 ft., the volume of free air per minute 1,000 cu. ft., the initial air pressure 1001b. per square inch, and the permissible pressure drop about 5 per cent. Then, following a vertical line from L = 2,000 to V = 1,000, a horizontal line to P = 100, a vertical line to drop = 5 per cent., it is found that a 4-in. pipe is the nearest commercial size available, although the drop for this size is about 6 per cent. The accuracy of the formula is dependent upon the assumption of a correct value of K, for which, however, there is very little reliable data available. In a recent paper read by Mr. Sam Mavor before the Institution of Mining Engineers at Leeds in September 1915,1 attention was drawn to considerable discrepancies between the pressure drops as calculated from various theoretical formulae and the actual pressure drops obtained on tests carried out by himself. If these tests are reliable, it would seem that the generally accepted formulae are liable to considerable error, and that to attempt to obtain exact results by long and complicated calculations is a waste of time. A labour-saving device such as this transmission diagram for air should, there- fore, prove a great convenience. * “ Entropy Temperature and Transmission Diagrams for Air,” University of Illinois Bulletin, 1913, No. 63. 1 See Colliery Guardian, September 17, 1915, p. 573; September 24, p. 622; October 1, p. 673. Partnerships Dissolved.—The London Gazette announces the dissolution of the following partnerships :—J. Gunn, A. H. Gunn, and E. Gunn, coal merchants, coal exporters, ship brokers, and agents, at Mountstuart-square, Cardiff, under the style of J. and M. Gunn and Company; A. E. White, A. S. White, and F. G. White, engineers, boiler makers, and ship builders, at the Vectis Works, Cowes, Isle of Wight, under the style of W. White and Sons ; H. W. Robinson and C. LI. Loyd, engineers, at Keen’s-road, Croydon, Surrey, under the style of the Royle Engineering Company: H. McIntyre and G. Jones, engineering contractors, at Albert- buildings, Preeson’s-row, Liverpool, under the style of McIntyre and Jones. Exports and Imports of Mining Machinery.—The value of the imports of mining machinery during the last month of the year was £5,047, against £3,376 in December 1914, and that of exports £42,854, against £48,800 in December 1914. It should be mentioned that these figures do not include prime movers or ’electrical machinery. According to destination, the value of exports was as follows :— December. To— 1913. £ 1914 £ 1915. £ Countries in Europe 6,452 ... 4,141 .. . 1,693 United States of America .... — 54 .. 269 Countries in S. America... . 3,915 . 2,364 .. . 1,665 British South Africa 21,702 ... 17,114 .. . 25,967 „ East Indies 12,399 ... 6,894 . . 3,329 Australia 2,102 893 .. .. 2,639 New Zealand 1,149 452 .. . 1 003 Other countries 12,256 ... 16,888 .. . 6,289 The following shows the values of prime movers exported, other than electrical :— December. — .— 1913. - 1914. 1915. £ £ £ All prime movers (except elec- trical) 752,483 ...489,935 .. .404,245 Included in the above were : Rail locomotives 204,751 ...183,742 .. .119,196 Pumping 53,067 ... 35,999 .. . 45,579 Winding 3,432 50 .. . 1,017 The Pressure of Gas in Coal Beds.* By N. H. DARTON. The gas confined in coal beds must exert a pressure closely proportionate to its relative volume, and as the volume is variable, the pressure varies accordingly. Observations made with tubes sunk deep in the coal show pressures ranging from several hundred pounds to tire square inch to those almost inappreciable. The causes of these variations are difficult to understand. As the gases are believed to be in pores and crevices in the coal, the pressure, as mining progresses, has a tendency to shatter the coal and throw it out from the face or rib. This condition is common, and in numerous instances men have been killed by masses of coal being suddenly detached. At the Marcinelle-Nord mine, in Belgium, in 1885, about 150 tons of coal was blown out in this way, and numerous large masses were thrown far from the face. Of course, at all times the coal in the face, rib, or pillar, is under stress from roof pressure and other sources, that has a tendency to break the exposed coal. This pressure is very great in places, especially where the beds dip steeply, some runs of coal being due to roof pressure independent of gas pressure. In such instances also, the rapidity with which the gas is given ofi may be due entirely to the sudden exposure of a large area of coal surface, and be no indication of gas pressure. Observations on pressure of the gas in coal have been made by numerous European observers; and during the investigation in Southern Illinois, the author made a number of tests of pressure in the solid coal. Gas Pressure in Coal Beds in England. The first extended investigation of this kind was made by Wood, in 1880, in English coal mines. The pressures ranged from 28 lb. to 461 lb. to the square .inch. They were measured in tubes tightly tamped in holes 3-1 ft. to 47 ft. deep that had been bored ahead from working faces into the solid coal. The volume of gas was also determined. The collieries were the Hetton, Epplo.on, Boldon, and Harton, all old ones, working bituminous coal 3 to 6 ft. thick. The workings were 1,228 to 1,268 ft. deep. In conducting the tests, a |in. pipe was inserted in.o the hole to within a few feet of its end. At this inner end was screwed a nut holding a washer nearly the diameter of the borehole (about 2 in.). The outer end was fitted with a collar, and between the collar and the nut were placed, first, a metal socket, and then several soft-rubber washers that could just be forced into the hole. When the nut was screwed up, a fairly tight fit was effected, but the final sealing was made by filling the space around the socket with Portland cement or oakum in cement. In some eases a wooden plug was was also forced into the socket. A gauge was attached to the end of the tube for reading pressures, which were observed hourly in most cases. In the deeper holes alternations of rubber washers and wooden plugs were used. The highest pressure was obtained at the Boldon mine, where the gauge reached 461 lb. in the deepest hole (32 ft.), which is about 84 per cent, of the pressure that would be due to a column of water the same height as the thickness of the cover. This mine had been opened only 11 years, whereas the others were four or five times ■as old, and this difference may account for the higher pressure. However, there were great differences in pressure in near-by holes, and not always in direct relation to their depths, although some of the results seemed to show a fairly close relationship, the ratios of the pressures per square inch being proportional to the square roots of depths. The direction of hole in rela- tion to cleat seemingly had no effect on the pressures. Mallard reviewed the results of Wood’s observations and deduced from them some hypotheses as to certain of the conditions of gas pressure. He compared the gas to water in a porous stratum, and believed the same laws of pressure and movement are applicable to it. He showed that under this condition the gas under high pressure back in the solid coal flows toward the face and to the spaces in the test holes, and the outward diminu- tion of pressure can be expressed by a mathematical formula. He explained the great variation in pressure from place to place by variations in original gas content, in porosity of coal, and in conditions that have permitted gas to escape from the bed. With gas confined in the coal under favourable conditions for its retention, and a thick mass of strata above, the pressures should be expected to rise to 450 lb. to 600 lb. to the square inch or even higher. In general, the highest pressures would be found in coal that presents the lowest coefficient of permeability. However, the pressure observations by Schorn, Watteyne, and Maquet do not give results in accordance with the Mallard formula. Gas Pressure in Coal Beds in Wales. The British commission on accidents in mines (1886) had some interesting pressure tests made in South Wales. At the mine of ;he Harris Navigation Company, the deepest steam coal workings in South Wales, a 30 ft. test hole bored at a depth of 2,133 ft. developed a pressure of 150 lb., but found little gas. A second hole bored at a depth of 2,169 f.. was 26| ft. deep, and showed pressure of 1161b. The volume of gas was 0'287 cu. ft. an hour. At the Merthyr Vale mine borings, 41 ft. 2 in. and 49 ft. 9in. at depths of 2,400 and 900 ft. showed pressures of 170 1b. and 280 1b., respectively. At the Celynen mine, near Newport, holes 42 ft., 47 ft. 10 in., and 20 ft. 3 in., at depths of 1,058 ft., 1,480 ft., and 1,500 ft., gave maximum pressures at 1291b., 430 1b., and 318 lb. The gas volume was at the rate of about 38 cu. ft., 0*4 cu. ft., and 36 cu. ft. in 24 hours. A test made to show pressure conditions in face of the coal * From Bulletin 72 of the U.S.A. Department of the Interior, Bureau of Mines. is described in the report of the British commissioners. A 4-in. iron tube was tamped into a bore hole 18in. deep, and fitted with a tight piston. A pressure gauge was attached to the side of the tube where it entered the coal. It was expected that when the piston was drawn out a partial vacuum would be indicated, but in most testis the gas was given off so rapidly under the reduced pressure that the gauge showed no change. When the piston was pushed in again the gas was forced back into the face, but escaped from crevices to a distance of 4 ft. to 5 ft. about the hole, where, on being ligh.ed, it appeared as numerous small flames. Gas Pressure in Coal Beds in Belgium*. Schorn, Watteyne, and Maquet in 1885 to 1887 investigated gas pressures in Belgium in an area where the beds are considerably disturbed. One series of tests was made by Watteyne in shaft 7 in the Belle-Vue mine at a depth of 2,264 ft. in the Petite Chevaliere, a very gaseous bed. The first holes were made in and near the face of a rock tunnel about 9 ft. from the steeply dipping coal bed. The holes passed through all of the coal, a. this point about 6 ft. thick, and iron tubes were set in the holes through the rock and tamped in. The holes were drilled at various angles, but were grouped in a few square yards. A pressure of 555 lb. was obtained in one hole. The results of the observations show :—(1) That holes did not affect the pressure at near-by holes, except possibly where there were some open cracks between; (2) that uncovering the coal so as to let out a great volume of gas from a large surface had no immediate effect on holes a few yards away; (3) that even the pressures observed, as high as 6371b., do not represent the full pressure of the gas disseminated in the coal if the volume of this gas is four times that of the coal; (4) that the gas is irregularly distributed in coal beds even within short distances; (5) that no ‘law can be applied to this variation in pressure; (6) that the pressure which is established slowly is due to the gas that accumulates little by little in the part of the beds nearest the boring, and so establishes an equilibrium against the real pres- sure of the gas in the bed, the resistance of the com- pact coal sustaining part of the pressure. Ghysen, in reviewing the results, remarked that the pressures observed at various places do not indicate the danger of gas outbursts, for in the very dangerous bed “ Epuisoire,” in the Agrappe mine, the pressure was less than 15 lb., whereas in the only slightly gaseous beds at Grand Hornu a pressure of 60 lb. was found, and at Marihaye a pressure of 2251b. was observed. In boring the holes it was noted that those holes showing highest pressure did not give the most gas before being closed, and that most of the gas was liberated in the front part of the hole, possibly because boring cracked the coal in advance, and so facilitated the libera- tion of gas. It was established by these tests that the gas pressure, after attaining a maximum, drops slightly, and then remains constant. This may indicate that coal near the hole later absorbs or drains gas from more dis- tant points where there is higher pressure, and so estab- lishes an equilibrium. Time is an important factor in the manifestation of the pressure. It was found that when a tube was opened, and then re-closed, the pres- sure was attained more rapidly than when the experi- ment was started. Some fluctuations in the pressure curves appear to indicate waves due to movements of gas through the coal bed. Maquet made tests of gas pressure at the Beaulieus art mine at Fontaine 1’Eveque in the Joseph coal bed in September 1886. The bed dips 40 degs., and, includ- ing partings of carbonaceous shale, is about 5 ft. thick. Two holes were bored into the bed at a depth of 1,631ft. at the face of a gangway in the mine, one directed to the north-west, and the other to the west, No. 2, being south of No. 1. Iron tubes were tightly tamped into the holes, and were provided with pressure gauges, which were read at intervals for 11 days. The gauge on hole 1 soon indicated a pressure of 1871b., and after several days 1151b. was shown on the gauge in hole 2. During the experiment the rise or incline was driven rapidly upward from the gangway, and in such manner as to cause gas outbursts of moderate volume at several points. The pressure in the two holes was affected in a pronounced but varying’manner, missing rapidly at first, and becoming much higher in hole 1 than in hole 2. On the second day the pressure (187 lb.) was released in the. tube in hole 1 for several minutes without affecting the pressure in hole 2, which was then gradually rising. This experiment tried at other times caused no oscillation in pressure in either tube, and the original pressure was quickly regained. On the second day a gas outburst occurred, detaching about 1,700 cu. ft. of coal, flooding the neighbourhood with gas, and stopping observations for 16 hours. Two hours before this outburst the pressure in hole 1 dropped suddenly, and continued to fall for several hours after the outburst, till finally on the next morning it reached 50 lb., and then ceased falling. In hole 2, the outburst had no effect on the steady rise of the pressure, which finally reached 1201b., the last few pounds by a rapid rise. Soon after there was a second gas outburst much like the first one. The pressure in hole 2 at that instant suddenly dropped from 120 to 441b., but the higher pressure was regained soon after the outburst. Mean- while, the pressure in hole 1 showed a slight fall three hours before the outburst, then remained stationary, and immediately after the outburst rose again. After a short time it became 12 lb. higher than on the previous day, and then gradually sank, and remained stationary a: about 341b. until near midnight, when it sharply descended to 22 lb. during a third gas outburst in the rise.