308 REID— SEISMOLOGICAL NOTES. [April 24. 



produce a " steady mass," that is, a heavy mass that would remain 

 at rest in spite of the movement of its support ; and by recording, 

 either directly or through magnifying levers, its movement relative 

 to the ground, the hope was entertained that the actual movement 

 of the ground would be obtained. But the hope was futile. Every 

 seismograph consists essentially of two parts : a heavy mass adjusted 

 in a greater or less degree to a condition of neutral equilibrium, 

 and the drum or other surface on which the record is made. If 

 the mass could be adjusted absolutely to neutral equilibrium and 

 could be kept in that condition in spite of the movement of its 

 support, it would remain at rest, and would record the true move- 

 ment of the earth ; but the size of the recording apparatus is limited 

 and in order that the record should be made on it, the heavy mass 

 must remain pretty closely in one position, which is practically in- 

 compatible with neutral equilibrium. It was found necessary to 

 keep the mass in stable equilibrium although the force brought into 

 play by a small displacement might be very small. If displaced the 

 mass would, therefore, vibrate about its position of equilibrium with 

 a period of its own ; and the record of every earthquake is the 

 combination of the earth's movement with that of the heavy mass; 

 and if the period of the vibrations of the earth happens to approach 

 that of the heavy mass, the amplitude of the latter increases greatly, 

 and indicates a movement of the earth much larger than actually 

 occurs. We cannot deduce the movement of the earth from the 

 record except by a careful analysis based on the mathematical theory 

 of the seismograph. This, fortunately, has been worked out; but, 

 unfortunately, it is rather complicated, and it is only in compara- 

 tively simple cases that it can be applied without very great labor. 

 The earlier investigators also thought that all solid friction or 

 viscous damping reduced the sensitiveness of the instrument, and 

 that a long period of vibration increased it. Solid friction is indeed 

 always harmful and should be reduced as much as possible, but 

 viscous damping is a great advantage and simplifies the interpreta- 

 tion of the record. Remembering that every earthquake consists 

 of vibrations of many periods, a glance at figure 4 will show the 

 great benefit of strong damping. The curves show the magnifying 

 power of the seismograph so far as it depends upon the ratio of the 



