342 EAETHQUAKES. 



If we turn to the effects of large earthquakes as 

 exhibited on the land, I think that we shall find records 

 of phenomena which are only to be explained on the 

 assumption of an action having taken place analogous 

 to that which takes place so often in the ocean, or an 

 action similar to that exhibited by small earthquakes, 

 and artificially produced disturbances, if greatly exagge- 

 rated. 



The only explanation for the phenomena accompanying 

 the Lisbon earthquake appears to be that the short quick 

 vibrations which had ruined so many cities in Portugal 

 had, by the time that they had radiated to distant countries, 

 gradually become changed into long flat waves having a 

 period of perhaps several minutes. In countries like 

 England these pulse-like movements were too gentle to be 

 perceived, except in the effects produced by tipping up 

 the beds of lakes and ponds. 



The phenomenon was not unlike that of a swell pro- 

 duced by a distant storm. It would seem possible that 

 in some cases pulsations producing phenomena like the 

 ' seiches ' of Switzerland might have their origin beneath the 

 ocean, or deep down beneath the earth's crust. Perhaps, 

 instead of commencing with the 'snap and jar' of an 

 earthquake, they may commence as a heaving or sinking 

 of a considerable area, which may be regarded as an un- 

 completed effort in the establishment of an earthquake or 

 a volcano. 



From what has now been said it would seem that 

 earth pulsations are phenomena with a real existence, and 

 that some of these are attributable to earthquakes. On 

 the other hand, certain earthquakes are attributable to 

 earth pulsations. Some of the phenomena which have 

 been brought forward have only a possible connection 

 with these movements, and they yet require investigation. 



