Chap. 8] MAGNETIC METHOD 345 



The Swedish mining compass is probably the earliest magnetic prospect- 

 ing instrument/* consisting essentially of a light needle in jewel and stirrup 

 suspension, free to move in a horizontal and vertical direction. The case 

 is carried over the ground by three cords tied together above the instru- 

 ment. To compensate for the vertical component of the earth's field, a 

 small piece of wax may be attached to the south end of the needle. 



Not only the vertical component but also the horizontal intensity affects 

 the mining compass. Their action is somewhat involved, since the 

 normal vertical intensity may be compensated by gravity, while the 

 horizontal intensity is not. No actual measurement of H or Z is made. 

 The instrument is uspd qualitatively. In its practical application, it was 

 soon discovered that the maximum indication is not found directly over 

 an ore deposit (if, as is generally done, the vertical intensity is not fully 

 compensated). 



The mining compass may be considered a dipping (or dip) needle 

 with automatic meridian adjustment; hence, in eq. .(8-15), a = 0. Further, 

 since the pivot in the jewel is above the center of gravity, i = 7r/2 and 

 Di = ikf(H sin 77 — Z cos rj). For the gravity moment, D2 = —em'g 

 sin I cos 77 = em'g cos rj, where m' is the mass of the wax used to place 

 the system approximately horizontal. From Di ■{• D2 = 0, 



MZ — em'g 1 /„ em'g\ ,^ 00 \ 



For full compensation of the normal vertical intensity Zo , AfZo = em'g^ or 



tan7, = ^^° = ^. (8-386) 



XI XI 



The mining compass may therefore be considered a vertical magnetometer 

 with the horizontal intensity as scale value. Since H = Z cotan I, 



tan 77 = tan I ^ . (8-38c) 



Hence, this instrument furnishes the magnetic inclination multiplied by 

 the "relative" vertical intensity anomaly and therefore acts like the dip 

 needle. As the horizontal intensity may be written: H = Ho + AH, 



tanr? = -- ^^ „ ■ (8-38rf) 



Xlo "T ^-tl 



^'^ For illustrations see Fig. 510 on p. 41 of Fr. J. Berg's catalogue, Magnetometers, 

 and Fig. 31 in Eugene Haanel's. On the Location and Examination of Magnetic Ore 

 Deposits by Magnetometric Measurements, Dept. Int., (Ottawa, 1904). 



2^ For difference between "dip" and "dipping" needle, see p. 346. 



