102 WALKS AND TALKS. 



meantime we must all hold tightly to the facts and loosely to 

 the theories. The learner must here content himself with the 

 uncertamty r acknowledged by his masters. 



* -V* V ' '</ 



!xVlil! THE UNSTABLE 



PHENOMENA AND CAUSES OF EARTHQUAKES. 



WHEN men feel the earth beneath their feet growing un- 

 stable, the most paralyzing sense of insecurity seizes them. The 

 ground supports every thing; to the ground man intrusts his 

 .most elaborate and substantial structures, and when it fails 

 him the dismay is complete. Yet the solid earth has not only 

 been shaken by throes which have ingulfed cities and popula- 

 tions and mountains, but there is scarcely a moment when its 

 movements or its tremblings may not be felt by the delicate 

 means of research employed by modern science. The stability 

 of the solid earth is instability itself. 



The destructive shock lasts but a few minutes, or even 

 seconds. The successive vibrations which devastated Calabria 

 in 1783 were felt during barely two minutes. On the occa- 

 sion of the destruction of the city of Lisbon, in 1755 and the 

 loss of 60,000 lives, it was the first shock, lasting five or six 

 seconds, which caused the greatest damage. As to extent of 

 damage, Sicily, in 1693, and Calabria, in 1783, have been 

 among the greatest sufferers. Each, according to best esti- 

 mates, lost a hundred thousand lives. In Syria, Japan, and 

 the Sunda Archipelago, earthquakes are reported to have been 

 attended by still greater fatalities. In the year 526, more 

 than 200,000 people met with death at Antioch and the ad- 

 jacent towns. The volcanic eruption of Kra-kat'o-a, in 

 August, 1883, was attended by a sea-wave and earthquake 

 which, according to reports, caused the death of twenty thou- 

 sand persons. 



The motions which constitute an earthquake are various. 

 Sometimes they are vertical, either slow or rapid. More gen- 

 erally they are horizontal. In such case, they consist mostly 



