124 Rev. Mr Scoresby on the Uniform Permeability/ of 



ductors into the grooves at a a, and substituting in their stead 

 the pieces of wood ; by this means the magnets are retained 

 firmly in their places, and the whole apparatus kept together. 



Whilst thus retained in their case, the powers and the focal 

 position of both bars together are determined, so that their con- 

 dition in practical operations is exactly the same. A table of 

 their powers (after the manner of that given in page 115) may 

 then be calculated, and the bars forthwith employed as occasion 

 may require. 



But I proceed with some suggestions for the practical appli- 

 cation of the foregoing principles and investigations in mining 



In all cases where both the level and direction of the work- 

 ings or mines are known to be the same, or in which the differ- 

 •ence can be satisfactorily ascertained, the application of the 

 proposed process for determining the thickness of interposing 

 substances, will be sufficiently plain from what has already been 

 stated ; for, in such cases, a single observation of the deviation 

 produced by the action of one or more magnets will be sufficient 

 to show the distance betwixt the magnet and the compass. 

 When the direction of the separating wall lies nearly north and 

 south, so that the compass does not require any directing mag- 

 net, the table of deviations already calculated for the magnets 

 in use will enable the miner generally to judge with sufficient 

 -accuracy of the distance, without the trouble of bringing the 

 magnets round to make the corresponding experiment*. And 

 even where a directing or neutralizing magnet is employed, it 

 would not be difficult to come at very satisfactory results by the 

 employment of the smaller set of proportional magnets (propor- 

 tional as to dimensions and directive power), so as generally to 

 render it unnecessary to remove the bars from one mine to the 

 other. For if the two pairs of bars were constructed at the same 

 time, of the same quality of metal, and of similar temper, there 



• Should the proposed plan of measuring the thickness of rock, &c. be 

 brought into general practice, it would be useful and important to have tables 

 constructed of the form of that in page 35, shewing the ratio of deviations for 

 magnets of different degrees of energy. A series of tables, for instance, might 

 be calculated for every degree, or for every two degrees, of deviation at one 

 focal length, from 55 or 60 to 75 or 80 degrees. 



2 



