382 
POPULAR SCIENCE REVIEW. 
renders everyone acquainted with the fact that carburetted 
hydrogen gas, or rather a mechanical mixture of carburetted 
hydrogen, nitrogen, and carbonic acid, is constantly escaping, 
in some mines, from the coal, and that mixing with a certain 
quantity of atmospheric air becomes fearfully explosive. When 
a flame, such as that of the Davy safety lamp, is brought 
into an atmosphere containing fire-damp, its presence is in- 
dicated by alterations in the condition of the flame, and the 
practised eye can determine very readily the presence of the 
enemy. Sometimes this carburetted hydrogen escapes from 
the coal suddenly, but generally it escapes gradually from 
the seam, and, not unfrequently, insidiously increases to a 
dangerous degree without its being observed. To avoid this, 
and under all circumstances to indicate the presence of fire- 
damp, Mr. Ansell avails himself of the knowledge we possess 
relative to the diffusion of gases. His apparatus takes various 
forms : the most simple being a thin India-rubber balloon full 
of atmospheric air, having a ligature of linen bound round its 
equator to prevent its lateral expansion. This being placed in 
any part of the mine, it is proposed to connect it by a wire 
with any ordinary electrical bell at surface. As the balloon ex- 
pands upwards by the diffusion of the carburetted hydrogen into 
the air which it contains, it releases a catch, and connection being 
made by a simple mechanical arrangement with the electrical 
or magnetic apparatus, the bell is rung. After this signal has 
been given, the Fire Damp Indicator has only to be removed 
for a few minutes into good air, and it is restored to its original 
state. Another form of instrument is that of the aneroid baro- 
meter, a porous earthenware plate or a slice of Sicilian marble 
being substituted for the brass back of the box. This little 
instrument, which is only of the size of an old-fashioned watch, 
is carried by the colliery viewer in his waistcoat pocket. At any 
suspected place it is taken out, and the index gives the baro- 
metric pressure at that depth ; this is noted ; then, if any fire- 
damp is present, the index moves a certain number of degrees, 
thus indicating with precision the percentage of carburetted 
hydrogen present in the air. 
This instrument must be regarded as one of the most beauti- 
ful of the applications of science. It bears no very remote 
relation in its principles, to the lamp of Davy itself, and it 
supplements it in a manner which appears to promise the 
possible highest utility.* 
* It is important to notice here the facts j ust brought out by a well- 
devised series of experiments. The safety lamps, of all varieties, are found 
to be rendered unsafe by the improvements in ventilation. By increasing 
the velocity of the currents in the levels of a colliery, the explosive atmo- 
sphere is driven through the lamp, and the external mixture fired. 
