360 
POPULAR SCIENCE REVIEW. 
piston slides, and a tuning-fork of definite pitch attached to 
a block of wood, to which is fixed the tube. The tuning-fork 
is sounded, and the tube moved out, until the proper length 
is found, which is indicated by the resonator intensifying the 
sound of the fork. A light gas requires a longer resonator, and 
the density of the gas can be read off the scale, and two per 
cent, of fire-damp mixed with common air, it is said, can be 
detected. 
The writer of this paper has recently, in letters to the Times 
and Colliery Guardian , suggested still another means of detect- 
ing the presence and volume of fire-damp. Taking advantage 
of the well-known and, to the miner, disagreeable whistling or 
blowing of gas issuing from the pores of the coal, he has sug- 
gested that a telephone (or microphone) be placed against 
steady blowers of gas, and that these receivers be connected with 
deliverers in the colliery office, which might be made to speak 
to a phonograph, automatically worked by clockwork, so that a 
continuous record of the quantity of gas evolved from these given 
test points might be kept and studied above ground. 
When it is remembered that the weight of the atmosphere 
balancing 30 inches of mercury is 2120*25 lbs. on each square 
foot of surface, and that a fall of one inch in the barometer 
indicates that the air resting on a square foot is 70*68 lbs. 
lighter, it will be readily understood how a sudden fall of the 
barometer may indicate a relief of the pressure, which has pre- 
vented the inflammable gases in the coal from emerging, and 
which, removed, allows them to pour into the mine. 
The relation of changes of atmospheric conditions to colliery 
explosions have been carefully studied by Messrs. Scott (Me- 
teorological Office) and Galloway (Inspector of Mines), with 
the result that 48 per cent, of the accidents might be attributed 
to changes of barometric pressure, the explosions generally 
taking place during the first fall after a long calm, and seldom 
until two or three days after the minimum has been reached. 
23 per cent, were attributed by them to the temperature of the 
atmosphere, the first hot days of spring being especially marked 
by accidents. When the outside temperature rises, so as to 
equal that underground, all natural ventilation ceases, and if 
the mechanical means provided have not a wide margin, an ex- 
plosion results. 
A high surface temperature, by reducing the quantity of air 
passing through the workings, allows the accumulation of gas on 
sensitive points ; while a very low surface temperature, by drying 
the coal dust in the mine, tends to make a mixture which is an 
exceedingly explosive vehicle, even when only one-seventh of 
the quantity of fire-damp is present, which is ordinarily ex- 
plosive with air. That this is often a cause of accident, there 
