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Choose a Page:Why do volcanoes erupt?*What is a seismograph?*How do we measure wind speed?What is a hurricane?Where does the wind come from?What causes lightning? What is a seismograph? A seismograph measures earthquakes. When an earthquake occurs, its hanging arm shakes, and the pen marks the paper on the revolving drum.   Previous: Back Book: Best Ever Q&A Section: Our World Chapter: Natural Forces