MINING. 295 



which the azimuth angles are to be read off by aid of a vernier 

 and lens on either side. The graduated ring is divided into hours, 

 degrees, and half-degrees, and, with its attachments to the needle, 

 is of aluminium. If the needle is to be used at all, it is indispen- 

 sable for accuracy that its many irregularities should be met by 

 due observation and precaution. It is usual in our mines to 

 ignore the secular variation or " declination " of the magnetic 

 needle, and where a plan is comprised of surveys added in suc- 

 cessive years, there can thus be no pretension to rigid accuracy. 

 At Przibram, in Bohemia, a meridian line is laid down in the map 

 office (inarkscheiderei) ; and observations are made several times 

 before and after noon, to determine the diurnal variation, which 

 differing in amount at different localities, may also be a fruitful 

 source of error. 



The same compass which has been employed in the survey is 

 upon this system usually fitted into a frame serving the purpose of 

 a protractor, and is thus applied direct to the plotting of the 

 survey on the plan. 



In some few cases of uncertain holing from one excavation into 

 another, a delicately poised needle has been applied to respond 

 to the action of a powerful magnet on the other side of a rocky 

 barrier. It would be interesting to have particulars of the nature 

 and results of such experiments. 



There is, lastly, another application of the magnetic needle 

 viz., to the discovery of deposits of magnetic iron ore. This in 

 Sweden, where it is often used, is in the form of a dipping 



needle. 



LIGHTING OF MINES. 



When towards the end of the eighteenth century great difficul- 

 ties were experienced in the working of such seams of coal as are 

 termed " fiery," from their giving off fire-damp, many devices of a 

 more or less scientific character were tested for giving light 

 without danger of explosion. The reflection of the daylight on a 

 series of mirrors, the phosphorescence of decaying animal matters, 



