Lesson 42 Recording an earthquakeFirst listen and then answer the following question.What does a pen have to do to record on paper the vibrations generated by an earthquake?An earthquake comes like a thief in the night, without warning.It was necessary, therefore, to invent instruments that neither slumbered nor slept.Some devices were quite simple.One, for instance, consisted of rods of various lengths and thicknesses which would stand up on end like ninepins.When a shock came, it shook the rigid table upon which these stood.If it were gentle, only the more unstable rods fell. If it were severe, they all fell.Thus the rods, by falling, and by the direction in which they fell,recorded for the slumbering scientist the strength of a shock that was too weak to waken him, and the direction from which it came.But instruments far more delicate than that were needed if any really serious advance was to be made.The ideal to be aimed at was to devise an instrument that could record with a pen on paper, the movements of the ground or of the table as the quake passed by.While I write my pen moves, but the paper keeps still.
