of all known Substances to the Magnetic Injiuence. 119 



Another important application of the proposed method of as- 

 certaining distances, is in the working of mines. When ap- 

 proaching an old working from a different shaft, or the vergQ of 

 an adjoining mine — perhaps a different property, — many re- 

 sons might exist, such as an accumulation of water, " fire- 

 damp," &c. why the separating wall should not be perforated ; 

 whilst, on the other hand, the value of the ore or bed of coal 

 might be such as to render it desirable to approach as near as 

 consistent with safety. No means, however, heretofore known, 

 would enable the miner to ascertain with accuracy the thickness 

 of the interposing wall ; whereas, by the proposed method, when- 

 ever the two sides of the wall were attainable, the thickness, 

 within given limits, might easily be determined. And the cases 

 are by no means unfrequent, in which a magnet, under proper 

 precautions, might be safely carried into an old working con- 

 taining fire-damp or water, though the breaking through into 

 that working might be attended with most mischievous or even 

 dangerous consequences. 



In mining beneath the sea also, or under the bed of a river, 

 the same principle (substituting a dipping-needle for the com- 

 pass) may be easily applied, so as to determine the thickness of 

 the stratum of earth or rock forming the roof of the mine, and 

 indicative of the sufficiency of the protection against overwhelm- 

 ing waters. 



Many supposable cases might occur in engineering, or in or- 

 dinary architectural works, in which the principle before us 

 would have important application. Suppose, for instance, that 

 it is proposed to build a spire upon the tower of a church, or 

 any other heavy superstructure upon a given wall, but it is not 

 known, from the want of embrasures or windows in some of the 

 sides of the tower, whether each portion of the basement be 

 sufficiently strong to sustain the additional weight. Instead of 

 perforating the wall, where no ordinary means exist of ascer- 

 taining its thickness, which might be laborious, and perhaps a 

 blemish to the interior^ the magnet would readilv give the 

 thickness to the greatest desirable accuracy. In like manner, in 

 engineering, the thickness of walls, or beds of rock, not other- 

 wise determinable but by actual perforation, might, in most 

 supposable cases, be measured by means of the magnetic devia- 

 tions. 



