310 REID— SEISMOLOGICAL NOTES. ' [April 24, 



down very slowly, the curves show that the magnifying power for 

 vibrations of very short period is unity ; that is, the record gives the 

 true amplitude of the earth's motion ; for vibrations of longer period 

 the magnifying power rapidly increases, and when the ratio of the 

 periods is unity; that is, when the period of the earth's motion and 

 the free period of the seismograph are equal, the magnifying power 

 becomes extremely large. For still longer periods the magnifying 

 power again decreases and when the period becomes very long, it 

 becomes extremely small. Since, therefore, the vibrations of 

 various periods are differently magnified, it is quite evident that the 

 record of an earthquake would be greatly distorted, some vibrations 

 being unduly emphasized, and others unduly minimized. It is just 

 in this respect that damping is beneficial. Within limits, the in- 

 equality of magnifying power for various periods becomes less as 

 the damping ratio becomes greater ; and when the damping is great 

 enough to reduce the relative amplitude of successive swings in the 

 ratio of 8:1, the magnifying power is nearly uniform for all 

 periods less than that of the seismograph. A seismograph, damped 

 to this amount, and with a period as long as the longest of those 

 present in the earth's vibrations, would give a much truer representa- 

 tion of the earth's movement. 



The advantage of a long free period is not to increase the sensi- 

 tiveness of the seismograph but to increase the range of periods over 

 which its sensitiveness may be maintained. Contrary to a very 

 general belief, the magnifying power for vibrations of very short 

 periods is not affected by the amount of damping. 



(c) Suggestions for a National Seismological Bureau. 



The work of collecting information regarding earthquakes, and 

 studying this material is so extensive that it cannot be carried out 

 thoroughly except with the aid of the federal government. The 

 United States is almost the only country of importance which does 

 not give governmental aid to the study of earthquakes ; and, al- 

 though, fortunately, the larger part of this country is only subject to 

 occasional slight shocks, extremely destructive shocks have occurred 

 within our boundaries, and certain districts are frequently visited 

 by earthquakes which cause much damage. The study of earth- 



