398 
GARFORTH: FIEE-DAMP DETECTOR. 
of the chimney, when it simply acts Hke the Davy, as there is then 
only the gauze between the flame and external atmosphere. If, 
however, the lamp is fitted with a tin shield securely fastened to the 
middle flang-e of the lamp, the chimney and gauze are so protected 
that lamps of this description have been made to resist a velocity of 
51 feet per second. This velocity is more than is required under 
ordinary circumstances, but it is difficult to say what is required to 
resist the extraordinary conditions met with in mining, such as the 
concussion of air produced by a blown out shot, or after a heavy 
fall of the superincumbent strata, either of which might force a large 
volume of explosive mixture at a high velocity on to the lamp. 
Considering the cubical capacity of a large goaf as compared with 
the narrow roadways leading thereto, it may be imagined that a 
velocity of more than 50 feet per second results when a heavy fall 
takes place. Judging from the latest trials, it is thought that at 60 
feet per second no existing lamp is safe. The advantages of resist- 
ing a high velocity are undoubtedly very great, and will become of 
still greater value year by year in those deep and extensive mines 
where large volumes of air are required for ventilation and for 
reducing- the temperature of the strata at great depths. In my 
opinion, however, the advantages of the tin shield and double or 
trible gauze have been gained by shielding or placing a species of 
obstruction between the flame and external atmosphere. Such 
being the case, the old argument that the mine can be kept safer 
by using the Davy, which detects small quantities of gas, still holds 
good in a sluggish ventilation. The Davy is known to be most 
unsafe, for the reasons mentioned of not being able to resist velocity, 
and was some years condemned in Belgium ; and in many English 
mines. The last serious explosion in this country, happened at 
Altham, in Lancashire, by which 39 lives were lost, and a great 
number injured ; the accident was tj-aced beyond doubt to the use of 
the Davy, and since the explosion the Tin shield lamp has been 
substituted. 
From the foregoing remarks it would seem that in any existing 
safety lamp where one qualification is increased, another is propor- 
tionatelp reduced ; so it is doubtful whether all the necessary 
