MAGNETIC METHODS 



105 



netic surveys, except to establish an area value of dip, where a dip needle 

 was being used. In like manner, a map of nation-wide scope of the total 

 magnetic intensity would be of limited use. It would serve as a general 

 guide for work with one type of field instrument, the Hotchkiss Super-dip 

 magnetometer. With this magnetometer, as will be explained, measure- 

 ments of total magnetic intensity at stations are obtained. 



MILES 



LINES OF EQUAL MAGNETIC VERTICAL INTENSITY 



Fig. 33. — A portion of the chart for the United States showing 

 contours of vertical magnetic intensity in c.g.s. units as of January 

 1, 1925. Such charts are used to find the latitude and longitude 

 correction for magnetic field surveys. 



The charts of vertical intensity and of horizontal intensity are, how- 

 ever, necessary in magnetic field work with vertical and horizontal mag- 

 netometers. From them the change in the magnetic force per mile for an 

 area of a proposed survey is determined. This change, in gammas per mile, 

 is applied as a latitude and a longitude correction, as will be shown later. 



From careful measurements as above described at some 4300 stations 

 in the United States, separate maps have been prepared showing the dis- 

 tribution of the three magnetic elements. A portion of the chart for the 



