Descriptions and theories of a number of different seismographs developed particularly for recording of very long-period seismic waves are presented. These include (1) electromagnetic strain seismograph with galvanometer of 8 minutes period and photographic recording; (2) displacement transducer strain seismometer with resistance-capacitance network and short-period galvanometer photographic recorder or with ink-writing recorder; (3) electromagnetic pendulum seismometer with RC network having transfer characteristic of a long-period galvanometer recorder or a heated stylus visible writer; (4) electromagnetic pendulum with period increased tenfold or more using shunt capacitance; and (5) electromagnetic pendulum with condenser-lengthened period and triple RC integrating network recording with either heated stylus visible writer, ink writer, or short-period galvanometer photographic recorder.

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