EARTHQUAKES AND VOLCANOES. 



explained, can only produce vertical and oscillating motions, it has 

 been found, in some cases, that the ground has been affected by an 

 horizontal as well as vertical displacement. In some cases also 

 a gyratory movement of the ground has been observed, so that 

 after the shock, the direction of the walls of buildings, and the 

 relative bearings of fixed objects, such as buildings, trees, and 

 the directions of hills and valleys have been changed. 



19. Humboldt states, that although the undulations of the 

 surface of the ground, which take place in earthquakes, have 

 been ascertained with some degree of precision, their periods of 

 alternation have not been so well observed. In the city of Quito, 

 which stands at the foot of the volcano of Rucu-Pichincha, at an 

 elevation of nearly 10000 feet above the level of the sea, he often 

 felt strong shocks of an earthquake at night. Yet the buildings 

 of the city, including many lofty churches, fine cupolas, and 

 houses consisting of many stories, were very rarely injured by 

 them, although much slighter shocks damaged lower buildings on 

 the Peruvian plains. 



The natives of the country, who are accustomed to the phe- 

 nomena, many hundred earthquakes occurring during a single 

 generation, explain this difference by the greater or less rapidity 

 of the horizontal oscillation, which, according to their experience, 

 is far more destructive than the vertical or rocking motion 

 produced by regular undulation. 



20. The earthquakes which produce a gyratory motion of the 

 ground are the most destructive, and happily also the most rare. 

 After the earthquake which destroyed Riobamba in 1797, and 

 that which took place in Calabria in 1783, walls were changed in 

 their direction without being thrown down, rows of trees, which 

 were previously strait and parallel, were, after the shock, in 

 different directions, and even in curved rows. Fields were 

 changed in their relative positions, those in which two different 

 crops were growing having interchanged places. 



21. Humboldt was shown among the ruins of Riobamba a 

 place where the entire furniture and contents of a certain house 

 were found buried under the remains of another. Numerous 

 cases occurred in which heavy articles of furniture were trans- 

 ported several hundred yards from their original position, so that 

 questions of ownership were raised and brought before the courts 

 of justice. 



In this case the ground, rent in various places, was affected at 

 once by vertical and horizontal oscillations, so that while one part 

 was heaved upwards another adjacent to it was sunk downwards 

 and transferred horizontally under the former. Such tossing and 

 agitation of the surface presents a striking resemblance to the 

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