aU Jcnotmi Substances to the Magnetic hifluence, ^c. 323 



Case I. — When the Distance to he Measured is in an east and west 



direclion. 



Let W, Fig. 1. Plate V. represent the wall or septum of a build- 

 ing or a mine, lying nearly in the direction of the magnetic meri- 

 dian, the thickness of which is to be determined. The compass is 

 placed at C, and, previous to the application of the magnet, is 

 so adjusted in position, that the needle is made exactly to coin- 

 cide with the north and south line of the instrument. The 

 north pole of the magnet M, being then presented to the com- 

 pass, on the opposite side of the wall, will repel into the direc- 

 tion a a the north pole of the needle, the deviation of which is 

 to be carefully noticed. Then bringing the same magnet round, 

 and presenting the north pole in the opposite position, as at m, 

 it is moved backward or forward (in an east and west direction 

 from the compass) till the deviation produced in the direction 

 b b, is the same as before. Then the distance from the pole of 

 the magnet to the centre of the compass, in the latter position, 

 ought to correspond with the distance through the wall to the 

 centre of the compass ; for where the deviations under similar ac- 

 tions are equal the distances will be equal. 



Case II. — When the Distance to be Measured lies in an oblique or 

 parallel direction, as to the Magnetic Meridian. 



Fig. 2. Plate V. represents a wall or septum of a mine lying ob- 

 liquely to the magnetic meridian. The compass is placed at C, 

 with the north and south line parallel to the wall, the needle as- 

 suming the direction of 7i, s. A small directing magnet is placed 

 at D, and so adjusted as to deflect the needle from its meri- 

 dional position into a state of parallelism with the wall. The 

 needle is now made to coincide exactly with the meridional line 

 of the compass, by an adjustment in the position of the compass, 

 when, the magnet being placed beyond the wall, as at M, 

 the deviation, in the direction a a, is observed as before. Then, 

 whilst every thing remains the same, the magnet is brought 

 round to m, into the straight line drawn from the magnet, and 

 prolonged through the centre of the compass, and adjusted in 

 distance, so as to produce the same deviation, in the direc- 

 tion b 6, which, of course, gives the interval as in the former ex- 

 periment. 



Yy2 



