180 REPORT— 1873. 



exerted will be given off iu compressed air at a pressur* of three atmospheres 

 iuto the receiver ; and this pressure is sufficient for effectually working our 

 machiner)''. Some makers of air-engines offer to guarantee a much larger 

 product ; but I base my calculations upon the smaller yield. 



If the pressure be much higher than three atmospheres, there is a material 

 increase in the frictional heat disengaged by the act of compression. The 

 engines do not work wdth the same ease ; and the result of our experience is, 

 that at 45 to 50 lbs. the maximum point of economy is attained. Calculating 

 its cost and taking a 40-horse-power boiler to consume 10 lbs. of coal per 

 hour per horse-power, or 2 tons of engine-coal per day of 11 hours at 8s. per 

 ton at the pit, we have a cost of 16s. per day. 



It is safe to calculate that this boiler will drive an engine of sufficient power 

 to supply four coal-cutting machines, being 4s. per day for each machine ; 

 and each machine will cut more coal in any given time, and do it in a better 

 manner, in an ordinaiy seam, than twelve men ; it follows, therefore, that the 

 equivalent of a man's power exerted for a whole day in cutting coal, can be 

 obtained, out of compressed air, at a cost in fuel of but 3|cL 



Assuming, then, that this comparison is an accurate one, it may be taken 

 for granted that the objection to its use, on the score of cost, has no founda- 

 tion in fact. 



And considering its many and remarkable properties for employment in 

 coal-mines, it may be useful to dwell briefly on some of those peculiarities. 



It is a power from which, and under no circumstances, can an explosion 

 happen ; and when an escape from the pipes takes place, it is more or less 

 beneficial, and not in any wise injurious. 



At every stroke of the piston the air is discharged from the cylinder of the 

 coal-cutting machine at a temperature of about freezing-point, compressed 

 into one third of its natural bulk ; and it has been found that the working of 

 only one machine has had the effect of reducing the temperature at the 

 working face of the coal to the extent of two degrees Fahrenheit. 



Occasionally ice is formed at the escape valves of the machine, but with- 

 out producing any inconvenience to their working. 



Now any thing that will reduce the temperature of a mine is an inesti- 

 mable advantage. It diminishes the risk of explosion ; and by increasing the 

 velocity of the ventilating current, it renders the occupation of a miner more 

 tolerable and more healthy. 



In very deep mines the internal heat will probably be found to be so great, 

 that manual labour of an exhausting character will be unendurable ; but the 

 discharge of so large a volume of pure air at a pressure of three atmospheres, 

 and at freezing-point, must exert a powerful and highly favourable influence 

 under the peculiar circumstances. 



It is well known that the lives which are lost through explosions of gas are 

 far more numerous from the effect of the damp wliich folloivs the fire, than 

 from the fire itself ; and in many cases nearly, if not all, the sufferers have 

 died from this cause. 



There has been no case of fatal explosion within the experience of our 

 machine w^orkings ; and therefore we have no facts upon which absolute 

 reliance can be placed ; but we draw the inference, that where coal-cutting 

 machinery may be in general use in any mine where an explosion of gas does 

 take place, those who escape from the first effect of the fire will most pro- 

 bably be saved from death. 



At a lamentable accident in this neighbourhood about two years ago, 

 ■when thirty-one lives were lost, twenty- five or twenty-seven of those unfor- 



