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lupton: safety lamps. 
feet; if we take a very extreme case, the man might travel on a good 
road 750 feet per minute, and meet a current of 1500 feet, making a 
total speed of 2250 feet; to be perfectly safe, therefore, a lamp should be 
capable of resisting this velocity for some time, but if, in addition to 
running against the wind, the man should swing his lamp violently, 
it would be brought against the air at a very much greater velocity. 
It is difficult to say at what speed a lamp might be swung. I find 
that I can give a three pound lamp 120 swings of five feet in a minute, 
making an average speed of 600 feet a minute, and it is probable that 
the maximum speed of swing exceeds this by half, making a speed of 
900 feet, this 900feet added to the previous velocity of 2,250 feet,makes 
a total of 3,150 feet. But this is only instantaneous, say a third of a 
second, because the backward swing of the lamp reduces the speed of 
current, so that a lamp ought to resist 3,150 feet for a third of a second, 
and 2,250 feet for a long period, say seven minutes. It must be remem- 
bered that the swing of the lamp might happen after it had been 
exposed for a long time to a rapid current. It may be worth while 
to consider if the conditions often occur in which a lamp is subjected 
to a rapid current of an explosive mixture ; the history of our coal 
mines gives a negative reply, it can only happen under one of three 
circumstances, as follows : — 
1. A very badly ventilated mine, where the return air course 
was foul to the explosive point, a condition of things almost, if not 
quite unknown in the last twenty years. 
2. A sudden issue of gas from stalls between pillars, caused by 
a large fall of roof ; a species of accident of exceedingly rare occur- 
rence. 
3. A sudden outburst of gas from the roof or floor of the mine, 
so powerful as to foul the main air courses to the explosive point ; 
this is also not a frequent occurrence, but is a more frequent cause, 
taking the last twenty years, of the fouling of a colliery 
than any other. I do not know the number of such outbursts, but 
probably in Yorkshire alone there is more than one every year, 
and perhaps two or three. If such an outburst happens and there is 
any exposed light on the return air side of it, an explosion will 
happen ; sometimes the intake is also fouled. 
