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. (1)_______________________. 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. (2)_______________________. While I write my pen moves, but the paper keeps still. With practice, no doubt, I could in time learn to write by holding the still while the paper moved. That sounds a silly suggestion, but that was precisely the idea adopted in some of the early instruments (seismometers) for recording earthquake waves. But when table, penholder and paper are all moving how is it possible to write legibly? The key to a solution of that problem lay in an everyday observation. Why does a person standing in a bus or train tend to fall when a sudden start is made? It is because his feet move on, but his head stays still.
