TEREESTRIAL MAGNETISM^ — HAZARD 249 



account, while others which seemed plausible at one stage of our 

 knowledge had to be discarded when our knowledge increased. 



Advances in other fields of science have been seized upon in the 

 hope that they might furnish a clue to the mystery of the earth's 

 magnetism. Cathode rays, the electronic theory of matter, the con- 

 stitution of the sun, and the probable condition of the interior of 

 the earth are all being studied as to their possible bearing on the 

 magnetic field of the earth. 



Gilbert's conception of the earth as a great magnet uniformly 

 magnetized about its axis of rotation, and subsequent modifications, 

 had to be discarded Avith the acceptance of a very high temperature 

 for the interior of the earth and the recognition of the demagnetiz- 

 ing effect of heat, coupled with the small amount of magnetic mate- 

 rial found in the visible rocks. At the same time the magnitude of 

 the anomalies (the departures from a uniform magnetization) indi- 

 cated the presence of large masses of magnetic material not far from 

 the surface. Recent investigations in various fields have suggested 

 the possibility that some of the properties of matter subjected to 

 very great pressure may be materially different from those observed 

 in the laboratory at ordinary pressures. Susceptibility to mag- 

 netization may be one of those properties, and the Department of 

 Terrestrial Magnetism, Avith the cooperation of the Geophysical 

 Laboratory of the Carnegie Institution of Washington, is arranging 

 a series of expeiiments designed to test the matter, 



Nippoldt, in 1921, returned to the idea of the earth as a mag- 

 netized sphere, because of the difficulty of providing a satisfactory 

 system of electric currents to account for the observed magnetic field. 

 He advanced the theory that the principal part of the earth's mag- 

 netic field consists of a nonhomogeneous magnetization of the 

 earth's crust down to a depth of about 20 kilometers. In addition, 

 there may exist for the earth's nucleus a magnetic field symmetrical 

 both about the axis of rotation and about the equatorial plane which 

 may have arisen in the same manner as the general magnetic field 

 of the sun. He assumed that the proportion of magnetite in the 

 earth's crust increases with depth below the surface, but that the 

 distribution is not homogeneous. He also assumed an inner core 

 composed of iron, nickel, and cobalt, as suggested by geophysicists, 

 with the possibility that it may be susceptible of magnetization in 

 spite of the high temperature, in view of the magnetic effects 

 observed on the sun by Hale. 



Adolph Schmidt points out that even on the assumption tliat at a 

 depth of 20 kilometers the earth's crust is composed entirely of 

 magnetite, the average susceptibility would not be great enough to 

 account for the observed conditions at the surface. 



