IGNEOUS AGENCIES. 155 



little about their cause. In fact, until about forty years 

 ago no attempt had been made to study them scientifi- 

 cally. Now, however, it is believed the foundations of a 

 true science of earthquakes' (seismology) have been laid, 

 and a true progress has been made. The basis has been 

 laid by Mr. Mallet, and progress has been made possible 

 by the use of self-registering seismometers. 



Frequency of Earthquakes. The slow development 

 of earthquake-science is not due to want of material, but, 

 as has already been stated, partly to the difficulty of the 

 subject, and partly to the terror produced unfitting the 

 mind for scientific observation. The earthquake catalogue 

 of Alexis Perrey records 18,000 in thirty years (1843- 

 1873),, or nearly two a day. When we remember that 

 three-fourths of the earth's surface is covered with the 

 sea, that a large portion of the land-surface is inhabited 

 by uncivilized races, and that even in civilized countries 

 many slight tremors are unrecorded, it will not seem 

 extravagant to say that, probably, there is not an hour of 

 any day in which the earth is not shaking in some portion 

 of its surface. 



Phenomena of an Earthquake. In brief, the phe- 

 nomena of an earthquake are : 1. Sounds, sometimes like 

 underground cannonading ; sometimes a hollow rumbling, 

 or clashing, or grinding. 2. Accompanying, or immedi- 

 ately succeeding, comes the movement of the earth, as a 

 slight tremor, or as a violent shaking ; in extreme cases, 

 so violent that the houses of whole cities are shaken down, 

 like card-houses of children, and bodies on the surface are 

 thrown up a hundred feet into the air, as at Eiobamba in 

 1797. 3. As to direction, the movement may be up and 

 down, or from side to side, or partaking of both, i. e., 

 obliquely, or it may be rotating or twisting, as, for ex- 

 ample, when chimney-tops are twisted about without 

 being upset, or wardrobes and bureaus turned about before 

 upsetting. 4. One thing is always observed and is of 



