EAKTH PULSATIONS. 341 



say, at the near station, it is clearly evident that the 

 period of the backward and forward motion rapidly decreased 

 as the motion died out. 



These illustrations are given as examples out of a 

 large series of other records, all showing like results. 



An observation which confirms the records obtained 

 from seismographs respecting the increase in period of an 

 earthquake as it dies out I have had opportunities of 

 twice making with my levels. After all perceptible 

 motion of the ground subsequent upon a moderately severe 

 shock had died away, I have distinctly seen the bubble 

 in one of these levels slowly pulsating with an irregular 

 period of from one to five seconds. 



Although we must draw a distinction between earth 

 waves and water waves, we yet see that in these points 

 they present a striking likeness. Let us take, for ex- 

 ample, any of the large earthquake waves which have 

 originated off the coast of South America, and then 

 radiated outwards, until they spread across the Pacific, to 

 be recorded in Japan and other countries perhaps twenty- 

 five hours afterwards, at a distance of nearly 9,000 miles 

 from their origin. Near this origin they appeared as walls 

 of water which were seen rapidly advancing towards the 

 coast. These have been from twenty to two hundred feet 

 in height, and they succeeded each other at rapid inter- 

 vals, until finally they died out as a series of gentle waves. 

 By the time these walls of water traversed the Pacific, to, 

 let us say, Japan, they broadened out to a swell so flat 

 that it could not be detected on the smoothest water 

 excepting along shore lines where the water rose and fell 

 like the tide. Instead of a wall of water sixty feet in 

 height, we had long flat undulations perhaps eight feet in 

 height, but with a distance from crest to crest of from one 

 to two hundred miles. 



