all known Substances to the Magnetic Injluence^ Sfc. 127 



of the board a a, when that edge is placed parallel to the inter- 

 posed substance, the thickness of which is to be ascertained. 

 The adjusting magnet is then fixed in its place by the thumb- 

 screw s, so as to secure a constant influence, whilst the parallel- 

 ism of a a with the wall of the mine is preserved ; and, in order 

 to facilitate and secure the preservation of that parallelism, a 

 deal plank may be placed directly across the mine against the 

 extremity of the working, and supported at the required level, 

 which would not only serve as a table for the compass apparatus^ 

 but having nailed upon it a straight batten near and parallel to 

 the edge, it would enable the experimenter to bear the compass 

 from side to side of an irregular working, and to preserve in 

 every part of the distance the parallelism of the straight edge 

 a a. Fig. 8. Plate I. illustrates this arrangement. 



Suppose T to be the working of a mine, and T' the approxi- 

 mating working, and it is required to ascertain the thickness of 

 the intermediate mass of rock R. The case of magnets M 

 being placed against the middle of the wall, in one of the heacL- 

 ways, the plank P is placed at the same horizontal level on the 

 opposite side, when the compass apparatus, guided by the 

 batten e e, being carried from side to side, will ghew the great- 

 est deviation in the position a. Let the magnets be now turned 

 with their opposite pole towards the compass, (the time for doing 

 which is to be indicated by signal, as about to be suggested), and 

 the deviations tried a second time across the mine. If the 

 greatest deviation be in the same position as before, that may be 

 assumed as the nearest point ; if not, the mean of the two posi- 

 tions must be taken. 



It is not necessary to describe the experiments more minute- 

 ly, nor to enter into the management of the apparatus for all 

 particular conditions ; for to do so would swell this paper to an 

 inconvenient length, whilst the details would be easily anticipa- 

 ted by a very little practice. It may be proper, however, to 

 mention, that where the deviations on the removal of the com- 

 pass continue to increase to the very extremity of the board, or 

 to the side of the headway, it will be necessary to cause the mag- 

 net to be removed in the opposite direction, so that the maxU 

 mum deviation may be satisfactorily obtained. 



