752 itEPORT — ISyi. 



means of assisting ventilation ; but it is very seldom tbat tlie air is used direct from 

 the mains for this pui'pose. A very interesting paper was read at the Newcastle- 

 upon-Tyne Meeting of this Association, in 1889, by Professor Alex. B. W. 

 Kennedy, on the experiments he had made in Paris upon the transmission of power 

 by compressed air, in which he states that an indicated efficiency of 31 per cent, 

 can be got from cold air, and 45 per cent, from air which has been heated after 

 compression. Tt is very doubtful, however, if in any compressed-air installation 

 used in coal-mining there is more than 30 per cent, of useful effect obtained ; in 

 many instances it is much less, as it is impossible in almost every case to heat the 

 air after it has passed into the mine ; and another source of loss is due to leakage, 

 caused in a great measure by the occasional upheaval of the ground disturbing the 

 pipes. It is thus obvious that compressed air is more costly than electricity ; but 

 up to the present time it is the only absolutely safe power which is capable of 

 being conveyed long distances underground in mines giving off fire-damp. 



A very large compressing plant has quite recently been erected in this district, 

 and another will very shortly be installed ; this latter will consist of two pairs of 

 tandem compound steam-engines, each pair having two high-pressure cylinders 22 

 inches in diameter, and two low-pressure cylinders each 40 inches in diameter, by 

 5-feet stroke. The air cylinders are 34 inches in diameter, one being placed behind 

 each low-pressure cylinder. Each high-pressure cylinder is provided with variable 

 expansion valves, which can be adjusted whilst the engines are working. The 

 independent condensing apparatus consists of a pair of engines having 10-inch 

 diameter cylinders, and 20-inch diameter air-pumps, by 2-feet 6-inch stroke, and 

 are so arranged that they can be worked together as a pair, or as single engines 

 and condensers. 



The pressure of steam at the boilers will be 150 lbs. per square inch, so that a 

 high degree of expansion may be obtained, and as the inlet and outlet valves of the 

 air-cylinders are of large area, and are perfectly free to act in sympathy with the 

 pistons in the air-cylinders, it is only reasonable to expect the' highest degree of 

 efficiency which can be obtained from steam-power applied to compressing air. 

 Each pair of air-compressing engines will be capable of developing at least 800 

 horse-power, or a total of 1,600 indicated horse-power in the installation. 



No provision has been made in this plant for compounding the air-cylinders 

 or the motors ; but it has been pointed out by Professor Elliott, of the Cardiff 

 University, in a very interesting and able paper read before the South Wales 

 Institute of Engineers, that great economy will result in compounding the air and 

 motors. Professor Elliott estimates the extra efficiency, under certain conditions 

 (if high pressure, as upwards of 11 per cent. Further investigation in regard to 

 defining the relative economy of using high- and low-pre.ssed air for underground 

 mining operations, and the relative cost of the plant adapted for the production 

 and application of each, is required before it can be definitely decided that air of 

 a pressure above four atmospheres, but with air-cylinders and motors compounded, 

 will result in real economy. It appears on tlie first blush as if we might look in 

 the direction indicated to secure a material increase in the effective power obtain- 

 able from compressed air. 



In some of our coal fields very hard seams or veins of coal are met with, and 

 various kinds of machinery have been devised to assist the coal hewer in .severing 

 the coal from the solid strata, and electrical appliances have in this class of 

 machinery been more or less successful. It appears to me, however, that there is 

 a want of simplicity about the majority of the machines which have come under 

 my notice, which will operate against their general adoption. 



In the conveyance of coal underground, from the face of the workings where it 

 i« loaded into the trams by the workmen, to the bottom of the shaft, several systems 

 of haulage are adopted, either worked direct by steam-power, compressed air, or 

 electrical motors, the principal of which are known as the endless-rope, main-and- 

 tail-rope, and main-rope systems. The two former systems are used where the 

 ground is either level or undulating, and where power has to be applied to haul 

 the trams or tubs in both directions, and the latter system is generally used where 

 there is a gradient in one direction, sufficient to allow the trams or tubs to run by 



