1. M. Bith , 1951 . Arkiv. for Geofysik ., 1 , 359 -393 .
2. M. Bith , 1952 . Pontiwae Academiae Scientiarum Smpta . Varia No. 12, p. 239 -276 .
3. Earthquake Seismographs and Associated Instruments
4. P. Bernard , 1952 .Un. Geod. Geophys. Int., Assoc. Seismol. et Phys. de l'int. de la terre, C.R. No. 10 (Brussels, 1951), p. 86 .