346 MAGNETIC METHOD [Chap. 8 



This equation illustrates plainly that the greatest deflections will be ob- 

 tained on the north side of an ore body (for, in the extreme case, 

 -AH = Ho, and 77 = 90°). 



The American mining compass, or meridian finder, an improvement of 

 the Swedish mining compass, was produced in the United States in 1860. 

 It was used for a magnetic survey by the Geological Survey of New Jersey 

 in 1880. The needle is free to rotate about a horizontal axis, the bearings 

 being mounted on upright aluminum strips capable of rotation about a 

 vertical axis. A modification of the American mining compass is Prof. 

 Louis' dipping compass. In this instrument the needle rotates in both 

 horizontal and vertical directions; the dip of the needle is observed on a 

 semicircular vertical scale which is weighed down and pivoted in line with 

 the axis of revolution of the magnet. The dial compass was first de- 

 scribed by William Borough in 1581, and is essentially a combination of 

 a sun dial and a compass. In application, the apparent time is determined 

 for the locality and the instrument so turned that the shadow of the dial 

 string falls on the dial division corresponding to this time. The plane of 

 the sighting attachment is then in the meridian and the angle between it 

 and the magnetic needle is the declination. Many magnetic surveys were 

 made with this instrument in the Michigan iron ore region in the early 

 days. 



The dip needle should not be confused with the dipping needle. The 

 dipping needle is an inclinator, or an instrument for the determination of 

 the inclination. In its magnetic system, the center of gravity is coincident 

 with the axis of revolution. The dip needle, however, has a counterweight 

 attached to its south side to compensate the inclination and is essentially 

 a vertical intensity instrument. If it were used at right angles to the 

 meridian, the deflections would depend on vertical intensity only. The 

 dip needle has been employed extensively in the Lake Superior iron and 

 copper country. In civil engineering it is frequently used for locating 

 buried pipes and other iron objects. As shown in Fig. 8-29, the dip 

 needle consists of a circular case with graduation in degrees and zero 

 division in the horizontal position. The needle is made of one or more 

 magnetic blades; the pivot is made of steel and supported by two agate, 

 cup-shaped jewels; and the counterweight is a brass rivet or a movable 

 rider. An arresting device is operated by a thumb lever at the top. The 

 meridian is first determined^^ by holding the case horizontal and allowing 

 the needle to adjust itself into the meridian. In this direction the case 

 is turned in the vertical position for the observation of the deflection from 



" N. H. Steam, A.I.M.E. Geophysical Prospecting, 345-363 (1929). 

 ^^ Fig. 8-29o shows a dip needle with automatic meridian adjustment (gimbal 

 suspension). 



