strong currents of air, a small valve may be placed at the inlet, as shown
in the drawing. This valve is made of sufficient weight to resist the force
of a strong current, and is only lifted from its seat by the pressure of
the hand on the mouthpiece. It will be apparent from the small size and
elasticity of the detecter that the test can easily be made with one hand,
and when the ball is allowed to expand a vacuum is formed within it, and a
sample of the atmosphere drawn from the breaks, cavities, or highest parts
of the roof, or, of course, any portion of the mine. When the sample is
forced through the tube near the flame, gas if present at once reveals
itself by the elongation of the flame in the usual way, at the same time
giving an additional proof by burning with a blue flame on the top of the
test tube. If gas is not present, the distinction is easily seen by the
flame keeping the same size, but burning with somewhat greater brightness,
owing to the increased quality of oxygen forced upon it.

I venture to claim for this method of detecting fire-damp among other
advantages: 1. The detecter, on account of its size, can be placed in a
break in the roof where an ordinary lamp--even a small Davy--could not be
put, and a purer sample of the suspected atmosphere is obtained than would
be the case even a few inches below the level of the roof, 2. The obtaining
and testing of a sample in the manner above described takes away the
possibility of an explosion, which might be the result if a lamp with a
defective gauze were placed in an explosive atmosphere. No one knows how
many explosions have not been caused by the fire-trier himself. This will
now be avoided. (Although lamps fitted with a tin shield will be subjected
to the same strict examination as hitherto, still they do not admit of the
same frequent inspection as those without shields, for in the latter case
each workman can examine his own lamp as an extra precaution; whereas the
examination of the tin shield lamps will rest entirely with the lamp man.)
3. The lamp can be kept in a pure atmosphere while the sample is obtained