326 EARTHQUAKES. 



CHAPTER XX. 



EARTH PULSATIONS. 



Definition of an earth pulsation — Indications of pendulums — Indications 

 of levels— Other phenomena indicating the existence of earth pulsa- 

 tions — Disturbances in lakes and oceans— Phenomena resultant on 

 earth pulsations — Cause of earth pulsations. 



The object of the present chapter is to show that from 

 time to time it is very probable that slow but large wave- 

 like undulations travel over or disturb the surface of the 

 globe. 



These movements, which have escaped our attention 

 on account of their slowness in period, for want of another 

 term I call earth pulsations. 



The existence of movements such as these may be 

 indicated to us by changes in the level of bodies of water 

 like seas and lakes, by the movements of delicate levels, 

 by the displacement of the bob of a pendulum relatively 

 to some point on the earth above which it hangs, and bv 

 other phenomena which will be enumerated. 



Indication of jpendulums. — Pendulums which have 

 been suspended for the purposes of seismometrical obser- 

 vations have, both by observers in Italy and Japan, been 

 seen to have moved a short distance out from, and then 

 back to, their normal position. 



This motion has simply taken place on one side of 

 their central position, and is not due to a swing. The 



