AND PREVENTING ITS EXPLOSION. 55 



and in some places is not more than four feet. On the other 

 hand, the occasional falling of the roof forms dome-like cavi- 

 ties above this height ; it is in these that the inflammable air 

 chiefly accumulates, and in the lower passages or workings, 

 where they are open, the draught of air must in general pre- 

 vent it from being collected. Hence the limits to the eleva- 

 tion of the lamp, and of course to the operation of the principle 

 on which the method is founded, are less than they at first ap- 

 pear ; though even the height of three, or three feet and a 

 half from the floor, while it is probably best adapted to the ne- 

 cessary illumination, will give the requisite security. 



A lamp with oil is more convenient than a candle, as requi- 

 ring no adjustment with regard to the wick ; and by the com- 

 mon contrivance of a plate with a screw-on the aperture, the 

 oil is prevented from being spilt, on any occasional inclination 

 of the lamp. The usual time of a miner's work is six hours; 

 the lamp, of the size just now mentioned, with fresh oil and 

 wick, burns seven hours. The miner, therefore, may take it 

 with him newly trimmed, and the lantern need never be open- 

 ed in the mine, by which any risk from the communication of 

 its flame to the surrounding air may be avoided. If it were ne- 

 cessary that it should burn for a longer period, it might, with- 

 out any inconvenience, be made of a larger size ; and the wick 

 might be made so as to admit of being raised by a contrivance 

 similar to that of Akgand's. All the joinings of the case or 

 lanthorn, it is obvious, ought to be as close as possible. 



By employing a metallic lantern, with a lens of very thick 

 glass in front, the risk from breaking, which is incurred when 

 a glass case is used, is avoided. This construction has other 

 advantages. It affords a great deal of light in the most favour- 

 able manner; the illumination being directed with less loss on 

 the space where it is required. Where the situation admits of 



it, 



