MINING. 293 



The dial was soon afterwards fitted with sights, usually made to 

 fold over the compass box, and by means of which the direction 

 of the draft would be taken upon vertical strings, or upon 

 candle flames. A semicircle graduated to twice 90 degrees is 

 often attached in the more modern instruments to one side of the 

 movable sight ring. The great irregularity and frequent lowness 

 of the levels or galleries in metallic mines led to the introduction 

 of the suspension-compass, which is to this day largely employed 

 in continental mining, and has shown itself capable in the hands 

 of careful observers of giving wonderfully accurate results. The 

 cord to which the compass is hung is tightened by screws which 

 have a ring handle, and it is usual, as soon as the line is duly 

 stretched, to take its inclination by aid of a light brass semicircle 

 with a delicately-poised plummet index.* 



Coming down into the present century, the rapidly diffused 

 application underground of cast-iron plates and rolled iron rails 

 introduced a new and serious source of inaccuracy into surveys 

 depending on the indications of the needle. Moreover, to say 

 nothing of the working of certain kinds of iron ore, there are 

 numerous mines in which the rock masses are capable of affecting 

 the magnetic needle in a notable degree ; and recent researches in 

 microscopic petrology have revealed the presence of magnetite in 

 many places where it was little expected to exist. Hence various 

 forms of the graphometer and theodolite have in the last half 

 century been largely introduced, instruments which, starting from 

 some defined line of bearing, may have the compass needle clamped, 

 and allow a survey to be carried out entirely by determining the 

 horizontal angles by aid of crossed wires in the telescope and of 

 a vernier, enabling the bearing to be read off to thirty seconds. 



In the efforts to introduce into the mines an instrument of 

 greater exactness than the old dial or circumferentor, it may be 



* Vide for figures and full explanation of these instruments, M. Combes, 

 "Exploitation des Mines." Paris, 1845. Lang von Hanstadt, "Anleitung 

 zur Markscheidekunst." Pesth, 1835. 



