COAL AND COLLIERY EXPLOSIONS. 
359 
vibration was distinctly heard, giving 28 beats a minute, or 
280 revolutions, which, multiplied by the area of the roadway, 
gave the quantity of air passing. 
Fire-damp mixed with air in the proportion of 1 to 4 
produces drowsiness when inhaled, and when increased to 2 
to 1, suffocation follows. When it only forms to -jL of the 
whole volume, the flame of the Davy lamp increases and 
gradually enlarges, and becomes surrounded by a pale blue halo, 
or sc cap,” as it is called by the colliers. When a or A of the 
volume is fire-damp, the mixture is highly explosive ; and with 
a a lamp will not burn for want of oxygen. 
The Davy is used as a sort of gauge to determine the presence 
and quantity by the colliers, and especially by the u fireman,” 
whose duty it is to examine the colliery before the men enter, 
for gas, as well as to fire their shots, during the day. The lamp 
is held close to the roof, and gas is indicated by the blue cap ; 
but the whiter the flame, the quicker it fires ; if the flame 
elongates, or flutters, it is gently drawn down so as not to be 
allowed to fire. If the lamp fills with flame, and continues 
burning, the collier pulls down the wick with the “ pricker,” and 
endeavours to extinguish it by plunging it into water, covering 
it with a woollen jacket, or dust and debris . If it cannot be 
extinguished, an explosion results. 
A Davy lamp is constructed of parallel iron wires, twenty- 
eight crossing each other in a square inch, leaving 784 aper- 
tures to each square inch of surface : when the gauze is new, or 
covered with oil, and raised to a red heat, it gives off inflam- 
mable vapour, which will ignite at that temperature, but this is 
only the case when the heat is applied from the outside, which 
is almost impossible in practice.* 
Professor Gr. Forbes has just described, at the British Asso- 
ciation Meeting at Dublin, an instrument for determining the 
quantity of fire-damp in mines. It consists of a metal tube, 
resonator, one inch diameter, and 15 inches long, in which a 
* Other experiments made by the North of England Institute of Mining 
Engineers show that “ an inflammable mixture of pit gas and air, moving 
at the rate of 8 feet per second, will explode against a stationary Davy lamp 
without a shield,” and at 12 feet with a shield, at 9 feet with a Stephenson 
lamp, or with a Clanny lamp. 
The comparative illuminating power of these safety lamps, using a wax 
candle, six to a pound, is as follows : — 
Average number of lamps required 
to equal a wax candle 
Davy’s lamp ...... 8 ‘00 
Stephenson’s lamp ..... 18*50 
Clanny’s (glass) 4*25 
Mueseler's (glass) 3*50 
