1. Schmidt, Ad., , , Beitr. Geophysik, , v. 41, , pp. 346– -358 , 1934, ; J. Bartels, Terr. Mag., v. 41, pp. 225–250, 1936.
2. Internat. Union Geod. Geophys., Assn. Terr. Mag. Elec, Bull.;Fisk,1930
3. McNish, A. G., , and E. A., Johnson, , , Terr. Mag., , v. 43, , pp. 401– -407 , 1938, .