104 EXPLORATION GEOPHYSICS 



in declination and horizontal intensity. Magnetic observatories have since 

 been established in nearly every country. Ambronnf lists forty-eight in his 

 book on geophysics published in 1928. 



Magnetic surveys have been completed by various agencies. These 

 surveys cover practically the entire w^orld, both land and vv^ater areas, from 

 latitudes 70° North to 60° South. They have been supplemented for high 

 latitudes by the very important magnetic measurements of polar expedi- 

 tions. In the last 40 years, particularly, improvements in methods of obser- 

 vation, coupled ^\\h an international comparison of instruments, have 

 insured high accuracy of results. 



Large scale magnetic studies in the United States have been made chiefly by the 

 Coast and Geodetic Survey, a branch of the Department of Commerce which was 

 established in 1843 and began magnetic work shortly thereafter. In 1899 that organi- 

 zation initiated a systematic magnetic survey of the entire country, which included the 

 occupation of at least one station in every county, the stations being situated about 

 30 to 40 miles apart. This task was completed in 1915. Since then reoccupation of 

 selected stations has been made to determine the changes in the earth's magnetic field. 



The Department of Terrestrial Magnetism of the Carnegie Institution in Wash- 

 ington, D.C., was inaugurated in 1904 under the direction of Dr. L. A. Bauer. It has 

 been active in completing the world magnetic survey. Magnetic measurements at sea 

 were carried out successfully, first on the "Galileo", a chartered sailing vessel, and 

 later on the "Carnegie", a sailing vessel with auxiliary steam power. It was especially 

 built for magnetic studies and was so completely free of magnetic material as almost 

 to eliminate the need for compass deviation corrections. 



Magnetic Charts. — Contours of equal value of the magnetic elements 

 can be drawn on the basis of Gauss' theory (of the distribution of earth 

 magnetism) from the evaluation of the observational data for the entire 

 earth. As displayed on special small scale maps, such contours are know^n 

 as isomagnetic lines. Isomagnetic lines, as drawn, are purposely smoothed 

 out and, insofar as is possible, do not take into account local magnetic 

 disturbances, except as the latter may affect the measured values at a 

 station used. 



There are a number of different types of such magnetic maps, or 

 charts, that cover the entire world, or in more detail, one country, as the 

 United States, England, or France, for example. 



These maps may show the following : a. lines of equal value of declin- 

 ation, called isogonal lines; b. contours of equal value of dip, known as 

 isoclines ; c. like lines of total value of magnetic intensity, which have no 

 special name; d. contours of vertical intensity component (Figure 33) ; or 

 e. contours of horizontal magnetic intensity. 



Such special charts giving declination are widely used in air and sea 

 navigation, and in land surveying. In general, however, isogonal maps 

 have found little application in magnetic exploration, although they have 

 been employed to some extent in the Lake Superior iron region. 



Maps showing the variations of dip are of little use in detailed mag- 



t R. Ambronn, Elements of Geophysics, McGraw-Hill, 1928 (translated by M. C. Cobb), 

 page 71. 



