324 PERMANENT EFFECTS OF EARTHQUAKES. 



motion of the ground begins very gradually, and it does not reach its 

 maximum for some seconds. An earthquake consists of many suc- 

 cessive movements, among which no single large movement stands 

 out prominently from the rest. The disturbance ends more gradually 

 than it begins. The area affected, duration and direction of move- 

 ment are very irregular and variable during the same earthquake. 

 Frequently the beginning of visible motion consists in a tremor of 

 short-period waves, about five to the second, followed by the principal 

 movements of one or two waves to the second. 1 



Distribution of Earthquakes. British earthquakes are always 

 slight : one-third of all that are known are recorded from the county 

 of Perth, and most of the others are also Scotch. 2 In Europe earthquakes 

 are common in all the regions of active volcanoes ; and they have 

 especially disturbed Calabria, the country about the mouth of the Tagus, 

 Agram, and several of the Greek islands, like Chios. Many districts 

 which experience earthquakes are free from volcanoes, though the 

 most violent earthquakes occur in volcanic regions. The destructive 

 earthquakes of Asia Minor and Syria are connected with regions where 

 volcanic action has become extinct ; but in South America earthquakes 

 are frequently connected with outbursts of volcanoes along the line of 

 the Andes. The ground disturbed by an earthquake as frequently 

 sinks as rises ; and there can be no doubt that the fractures produced 

 by undulation of the rocks, which must develop the character of 

 master-joints, are favourable to dislocation. The movements rarely 

 last more than a minute, and frequently only for a fraction of a 

 minute, though one at Tokio lasted four and a half minutes. The 

 areas over which the disturbance extends are extremely variable, the 

 most extraordinary record being that of the Lisbon earthquake of the 

 ist of November 1755, which affected the North of Africa and 

 Western Europe, and appears to have crossed the Atlantic and 

 travelled to the valley of the Mississippi. 



Effects of Earthquakes. Besides the production of cracks in 

 buildings in the lines of wave motion, tangents to which pass through 

 the centre of disturbance, we may cite as among the best-known per- 

 manent effects of earthquakes the formation of the Ran of Katch in 

 1819; the uplifting of the shore of Cook Straits ten feet in 1855 ; 

 and the well-known elevation of the Chili coast recorded by Darwin 

 in the "Voyage of the Beagle." 



Causes of Earthquakes. Earthquake action has sometimes been 

 connected with variation in atmospheric pressure, and with the attrac- 

 tion of the sun and moon. Thus earthquakes are more numerous in 

 mountain regions, like the Alps, than in lower regions. The circum- 

 stance that they are most numerous in winter would apparently indi- 

 cate that the radiation of earth's solar heat in winter causes contraction 

 of the rocks, resulting in dislocations which produce perceptible vibra- 



1 Memoirs of Science Department Tokio Daigaku. No. 9, Earthquake Mea- 

 surement, by J. E. Ewing, 1883. 



3 On the 22d April, 1884, an earthquake of some severity was experienced 

 about Colchester, and felt along a line running N.W. by way of Leicester. 





