INTRODUCTION 5 



magnetic inclination because it showed that the angle assumed by a 

 magnetized needle was due to the inclination of the earth's magnetic field 

 and not, as originally believed, to a mass which had been added during its 

 magnetization. 



William Gilbert, a physician and physicist contemporary of Queen 

 Elizabeth of England, conducted various scientific investigations and 

 experiments with magnets and magnetic bodies. His conception of the 

 earth as a giant magnet was very advanced for his time. His book "De 

 Magnete" was published in 1600. 



In 1722 George Graham discovered that the orientation of the compass 

 varies slightly throughout the day. Also, by the end of the 18th century 

 it was known that the earth's total magnetic intensity varies laterally over 

 its surface, and it became vaguely evident that there is some relationship 

 between structural geology and the inclination of the earth's magnetic 

 field. These observations were made by Baron von Humboldt, a German 

 scientist and traveler, in 1798-1803. The diurnal variations in the intensity 

 of the earth's magnetism were first noted by the French astronomer 

 Dominique Arago in 1827. To Karl Friedrich Gauss, who carried out 

 magnetic investigations in Gottingen in 1834, must be given credit for 

 working out methods of measuring terrestrial magnetism. The electro- 

 magnetic unit of magnetic force is named for him. 



As late as 1616, in Magnetical Advertisements, the compass was spoken 

 of as "the most useful and admirable instrument of the whole world, yet 

 blundering." In 1820, the English physicist Peter Barlow reported that 

 "half the compasses of the British navy are mere lumber and should be 

 destroyed." He built an improved one which remained in use until 1876, 

 when the present type was adopted. As the compass has developed so 

 have its uses. 



Exploration geophysics probably had its beginning about 1640 when 

 the compass was first used in the search for iron ores in Sweden. Theoret- 

 ical aspects of the work were propounded in 1843 by Von Wrede, who 

 suggested that local variations in the earth's field might be indicative of 

 buried magnetic materials. Magnetic measurements were conducted in 

 the Michigan iron district in 1873 by T. B. Brooks, in 1875 by H. Smock, 

 and in 1899 by H. L. Smyth. Numerous measurements were made with 

 these early, insensitive instruments. In 1879, Thalen published a detailed 

 account of magnetic technique and results. Investigations on placer gold 

 deposits were conducted in 1914 by Gibson, who carried out surveys in 

 Butte and Shasta Counties in California, using a Thalen-Tiberg mag- 

 netometer. Hotchkiss used a dip needle for exploratory work over the iron 

 ore deposits of Wisconsin in 191-5. 



Since the beginning of the century, numerous large scale geomagnetic* 

 investigations have been made by various organizations throughout the 



*The term gcovmgnctics as used here refers to the study of the magnetic properties 

 of the crust or outer layers of the earth. 



