THE EARTH. 73 



Tims, the vapours found beneath the surface 

 of the earth are very, various in their effects upon 

 the constitution ; and they are not less in their 

 appearances. There are many kinds that seem- 

 ingly are no way prejudicial to health, but in 

 which the workmen breathe freely; and yet in 

 these, if a lighted candle be introduced, they im- 

 mediately take fire, and the whole cavern at once 

 becomes one furnace of flame. In mines, there- 

 fore, subject to damps of this kind, they are oblig- 

 ed to have recourse to a very peculiar contrivance 

 to supply sufficient light for their operations. 

 This is by a great wheel, the circumference of 

 which is beset with flints, which striking against 

 steels placed for that purpose at the extremity, a 

 stream of fire is produced, which affords light 

 enough, and yet which does not set fire to the 

 mineral vapour. 



Of this kind are the vapours of the mines about 

 Bristol : on the contrary, in other mines, a single 

 spark struck out from the collision of flint and 

 steel, would set the whole shaft in a flame. In 

 such, therefore, every precaution is used to avoid 

 a collision ; the workmen making use only of 

 wooden instruments in digging ; and being cau- 

 tious, before they enter the mine, to take out 

 even the nails from their shoes. Whence this 

 strange difference should arise, that the vapours 

 of some mines catch fire with a spark, and others, 

 only with a flame, is a question that we must be 

 content to leave in obscurity, till we know more 

 of the nature both of mineral vapour and of fire. 

 This only we may observe, that gunpowder will 



