104 WALKS AND TALKS. 



Mallet, the center of disturbance of the Calabrian earthquake 

 of 1857 was seven to eight miles below sea-level. From this 

 point, the waves traveled in every direction, assuming posi- 

 tions like the concentric shells of successively larger spheres. 

 Dr. Oldham found the focus of the great Cachar earthquake 

 of 1869 in India, to be considerably deeper. 



It has been a common opinion, from ancient times, that 

 earthquakes are sometimes characterized by vortical or twist- 

 ing motions. The latest investigations, however, do not sus- 

 tain this view. Every position assumed by objects moved can 

 be explained by motions of a rectilinear, vibratory character. 



Sounds often accompany earthquakes, even when not coin- 

 cident with volcanic eruptions. Sometimes they resemble 

 explosions as of distant artillery; more frequently it is a 

 rumbling sound as of heavy vehicles moving over a city 

 pavement. I have myself experienced but one noteworthy 

 earthquake ; and that happened in Michigan and neigboring 

 regions on the 19th of September, 1884. It lasted about ten 

 seconds. The floor on which I rested was very perceptibly 

 vibrated, and a rumbling sound was extremely audible, like 

 that of a train of cars, with the beats quite rhythmical. 



Among the effects of earthquakes, though of a secondary 

 character, are the drying up of springs, the sudden increase 

 of their volume, the augmentation or diminution of their 

 temperature and the production of muddiness in the water. 

 Artesian wells are similarly affected. Sometimes the occasion 

 is signalized by the escape of mud, water, gas, or flames. Oc- 

 casionally, as in the Andalusian earthquakes of 1884, the 

 ground is rent open for considerable distances. During the 

 frightful disturbances of Calabria in 1783, the phenomena 

 of ground-ruptures ranked among the grandest and most fear- 

 ful effects of the catastrophe. Whole mountain sides, under- 

 mined by water, slid down in mass, and tumbled into the 

 plains below, covering all the cultivated ground. Cliffs fell 

 down in a body, and rocks opened, swallowing the houses 

 which stood upon them. At the western base of the granitic 

 chain of the peninsula, the ground affected by the shock was 



