284 THE BOTTOM OF TEE SEA. 



and the rock is less affected by it. For this reason, 

 where there is a tide, the operators await the period 

 when it is at its highest before firing the charge. 

 This precaution is more than ever necessary where 

 the rock is broken up, and it is sufficient to take 

 advantage of its natural crevices and employ a charge 

 cipplied in bottles. 



11. English Mines beneath the Ocean. 



Man's submarine labours are not limited to the 

 surface of the sea-bottom. Nature hides some of 

 her treasures beneath the sea as well as beneath 

 mountains. Coal, iron, tin, and other minerals are 

 often obtained from great depths. A vertical shaft 

 affords communication with the horizontal galleries 

 from which the mineral is extracted. At several 

 points of the English coast the miner does not hesi- 

 tate to carry his galleries beneath the sea, at the 

 risk of being drowned, if the least fissure permits 

 the ingress of water. But this danger is also en- 

 countered in ordinary mines, for the immense bodies 

 of water known to exist in the crust of the earth 

 would be much more than sufficient to destroy, in an 

 instant, the most gigantic subterranean works. The 

 enterprise is not, therefore, so hazardous as might 

 appear at first sight ; it presents in other cases about 



