THE SEISMOGRAPH AT THE AMERICAN MUSEUM 



By Edmund Otis Hovcy 



THERE are several kinds of seismographs or instruments for detecting 

 and registering the passage through the earth of the waves which 

 are caused by the disturbances whose results are famiharly known 

 as earthquakes, but all make use of the same principle of action — namely, the 

 inertia of a weight freely suspended or supported above the earth. When a 

 shock passes beneath the weight or "steady mass," as it is called, the earth 

 tends to vibrate back and forth without causing motion in it. In order to 

 obtain this freedom of motion in the instrument which was recently installed 

 in the Museum, a strong frame of angle iron supporting the steady mass 

 rests on a concrete pier whose base is firmly cemented to the solid rock. 

 The pier, furthermore, is entirely free from contact with the building or its 

 floors, so that no local vibration can be transmitted to the delicate register- 

 ing apparatus. 



The steady masses are of iron and lead and each weighs about 450 

 kilograms or 990 pounds. The}' are suspended in such manner that they 

 act as horizontal pendulums, so that their own plane of oscillation is tangent 

 to the surface of the earth. The horizontal axes of the masses, along which 

 they are free to move, are at right angles to each other, one lying true north 

 and south and the other lying true east and west. The differential motion 

 between the earth and the pendulums is what is recorded an<l measured by 

 means of a styhis that rests lightly upon the surface of smoked paper which 

 is drawn slowly under it, making a white line. Each stj'lus is connected with 

 its steady mass by means of a system of levers which ends in the center of the 

 pendulum. The steady masses would soon acquire a pendulum swing of 

 their own, hence a part of the system of levers is a " damper" of sheet alumi- 

 num close-fitting within a box. The observer can regulate the pressure of 

 the air against the aluminum sheet, thus checking the induced oscillation. 



The strip of recording paper is coated with lampblack and then put over 

 a pair of drums which are rotated at a uniform rate of speed by clockwork 

 mechanism arranged to run at the rate of fifteen millimeters (0.59 inch) 

 per minute. At the beginning of each minute the needle is raised from the 

 paper by means of an electromagnet connected with an accurate clock. 

 The break thus made lasts for four seconds and therefore is one millimeter 

 long and the successive breaks enable an observer to determine the time 

 when any part of the paper passed under the stylus. 



The heavier the steady masses, the greater the degree of magnification 

 of the actual movement of the earth that may be obtained and the greater 

 the degree of sensitiveness of the whole apparatus. The instrument at the 

 Museum being very large, it is possible by the varying of the relative lengths 

 of the levers in the system connected with the stylus, to vary from 130-fold 



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