690 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



The induction-spark, which, as we have seen, is produced in a 



FIG. 446. Dumas and Bcnoit's electric lamp for miners. 



rarefied medium or in a vacuum, though giving but a feeble light, 

 is nevertheless sufficient for the illumination of mines, and they 



FIG 447 Electro-magnetic apparatus for the miner's lamp. 



are thus adapted to be safety-lamps, such as that represented in 



