lupton: safety lamps. 
273 
It may be taken that the Stephenson or the Mueseler lamp, if 
well constructed, are safe enough for the general conditions of a mine, 
and that extra precautions are only required when there is danger of 
a great outburst. It is, however, difficult to say where this danger 
does not exist, if we except those mines mostly near the outcrop 
where firedamp is hardly ever seen ; in all the collieries where fire- 
damp abounds in the coal and in the strata there is a possibility of 
an outburst, and it is to guard against this possibility that the recent 
inventions have been directed. Having found that the source of 
danger is the velocity of the air-current, the most obvious cure is to 
put the lamp behind some shelter, — this is done by putting a Davy 
lamp into a tin can with a glass plate in front (see fig. 5), — with this 
protection the Davy lamp from being the least safe becomes one of 
the safest of all lamps. It has been exploded with a velocity of 2800 
feet; it has frequently resisted a velocity of 3100 feet. 
The Royal Commissioners consider that it may be implicitly 
trusted in a current of 2000 feet. But some forms of the tin-can 
lamp explode at 1200 feet a minute. But this is not satisfactory if 
we are to be perfectly safe. The next step then is to take the safest 
of the unshielded lamps, the Mueseler, and to put it in a case that 
will protect it from an air-current. (See fig. 6.) If this case is 
properly constructed it makes a lamp that will resist a current of 
2400 feet a minute with certainty, and will generally resist a current 
of over 3000 feet, and if the chimney is very carefully proportioned, 
and the bonnet also, it is possible that this lamp will resist a velocity 
of over 5000 feet a minute, and it is in fact absolutely safe. Mr. C. 
E. Rhodes says that the lamp is safe at 52 feet]a second, but I con- 
sider Mr. Rhodes' 52 feet is really 104 feet, owing to the contraction 
of the air-passage by the lamp. There are a variety of modifications 
of the form of shield, of the chimney, and of the gauze, for some of 
which is claimed a greater degree of safety, in fact that no velocity will 
explode them. There is a practical difficulty in testing lamps at 
high velocities ; a considerable force is required to produce the air- 
current, and large gas-holders. And also, some of the lamps are 
blown out in the testing-tube before the gas reaches them. In con- 
sidering how to make a safety lamp, it is natural to reflect that if 
