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The system consists of three long‐period seismometers and one microbarograph. Capacitive transducers and direct frequency modulation are used. The characteristics of the seismographs are: Resolving power, 1 m μ for periods >2 seconds; short‐term noise, <1 m μ; electronic drift ≈100 m μ/hour; dynamic range, greater than 80 db. Feedback is normally used to reduce drift. The pulse integrator provides an analog output. A reduction of aliasing is obtained by using the analog output to drive, through a high‐pass filter, a voltage variable capacitor connected in parallel with one transducer. Digitization is accomplished by using a buffered binaiy counter for each instrument. Sampling is done 20 times/sec. Results are stored on a ¾‐inch magnetic tape (speed 0.4 inch/sec). This tape can be used directly on a digital computer. Simple filters give responses ranging from those of a very‐short‐period instrument to that of a tidal gravimeter. Programs for searching, editing, filtering, and computing various kinds of spectrums have been written. The instrument is primarily useful for research rather than for routine work.

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