344 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



Two-thirds of the errors averaged 10 paces per mile, or 1 in 200, wliile the 

 maximum was 1 in 30. But this was better than the average for the season. 

 The observations at each station consisted in a reading- of the horizon- 

 tal and dip needles. When there was no local magnetic disturbance, the 

 horizontal needle would come to rest in the magnetic meridian, which in 

 this region is about N. 2° to 3° E., or almost coinciding with the true merid 

 ian. The dip needle, when held in the same meridian, would indicate the 

 index error. When, however, disturbing material was present, the horizon- 

 tal needle would point -to the east or west of the magnetic meridian, at an 

 angle determined by the direction of the resultant of the horizontal com- 

 ponents of the earth's and the local forces. The dip needle would come to 

 rest in the same vertical plane, at an angle with the horizon determined by 

 the amount and direction of the three forces, the whole pull of the earth's 

 force, the whole pull of the local forces, and the balancing weight, and in 

 general would show a downward deflection. After making and recording 

 the set of observations at a station, the party proceeded to the next, and so 

 on to the end of the day. At the end of each day, or as soon as possible 

 afterwards, the day's work was platted on a large-scale map, on which the 

 readings of the horizontal needle were represented by short arrows drawn 

 through the stations, turned east or west of the true meridian, as the case 

 might be, and carrying the amount of declination written at the arrow 

 point. The dip observations were laid off to scale iminediately below the 

 stations, measuring all from the same horizontal line, and the points thus 

 established were connected by a free-hand curve. 



SECTIOK T. FACTS OF OBSBRVATIOlSr AXD GENERAL PRINCIPLES. 



I. OBSERVED DEFLECTIONS WHEN THE STRIKE IS NORTH AND SOUTH AND 

 THE DIP VERTICAL. 



If a magnetic rock, striking north and south and dipping vertically, is 

 crossed by an east-and-west traverse, it is found, as the disturbing belt is 

 approached, say from the western side, that the horizontal needle points 

 toward the east 'of north, and that this easterly pointing gradually 

 increases to a maximum. Continuing east from the maximum point the 

 eastward declination decreases, and soon a station is reached at which the 

 needle points due north. Still farther east the declination changes to west- 

 ward, and soon thereafter reaches .a westward maximum, beyond which 



