296 SCIENTIFIC APPARATUS. 



and the sparks produced by a flint on the rotation of a wheel in 

 the "steel-mill," were all methods of obtaining but a gleam of 

 light, which in particular cases might nevertheless be welcome. 

 The latter apparatus was especially trusted to in many collieries, 

 but was at length, in several instances, proved to have been the 

 cause of an explosion. 



The introduction by Sir Humphrey Davy, in 1816, of a cylinder 

 of wire gauze, preventing the flame within from communicating 

 to the explosive atmosphere outside it, established a new era for 

 coal mining. The gauge at first recommended for the safety 

 lamp, 28 wires to the linear inch, or 784 apertures to the square 

 inch, remains that almost universally in use, though variations 

 may be noted in certain districts. The dimensions also remain 

 much the same as those first manufactured on the large scale, 

 although both length and diameter, and therefore the volume of 

 flame which they may sometimes have to contain, are in excess of 

 those in Davy's original trial lamps. 



But the number and variety of the safety-lamps which have 

 been produced since that period is so considerable, and many 

 amongst them prove to be so unfitted for practical use, that it may 

 be well in the small space at our disposal only to point out the 

 chief desiderata, and to mention those kinds which have been 

 admitted in large numbers to every day use. 



The simple Davy, as was pointed out by its illustrious in- 

 ventor, is no longer safe if exposed to a current of air of more 

 than five or six feet per second. Recent experiments have 

 shown that with a velocity of eight feet it will pass the flame; and 

 hence if exposed to a strong draught of foul return air, or to a 

 blower, or to the vibration of a wave of air impelled from a con- 

 siderable distance, it ceases to be a safeguard.* Another objec- 

 tion to the Davy is its insufficient light, offering a premium upon 

 working by unprotected flame. 



* See paper by Mr. "W. Galloway. Proc. R. Soc., June, 1874, and Trans. 

 North. Inst. Eng. 



