ON ITS EXTERIOR. EARTHQUAKES. 167 



Change of place, agitation, uplifting, and the produc- 

 tion of fissures, mark the essential character of the 

 phenomenon. We must distinguish between the acting 

 force which gives the first impulse to the vibration, and 

 the constitution, propagation, and enhanced or diminished 

 intensity of the agitation-wave. In the " Representa- 

 tion of Nature," I described those effects which present 

 themselves immediately to the senses, and which I had 

 myself the opportunity of observing for so many years 

 on the sea, on sea-like plains (the llanos), on elevations 

 of from eight to sixteen thousand feet, on the margin of 

 the craters of active volcanoes, and in granitic and mica- 

 slate districts distant about twelve hundred geographical 

 miles from any fiery eruption, in districts where at 

 particular periods the inhabitants no more count the 

 number of earthquake shocks than we in Europe count 

 the number of showers of rain, where Bonpland and 

 myself, in journeying through the forest, had to dismount 

 from our mules on account of their becoming unruly 

 from uneasiness at the earth trembling uninterruptedly 

 beneath their feet for fifteen or eighteen minutes. By 

 being so long habituated to these phenomena, (an 

 advantage shared subsequently in a still higher degree 

 by Boussingault,) one becomes better disposed for calm 

 and accurate observation, for collecting and collating 

 with critical care, at the time and on the spot, various 

 and perhaps discordant accounts, and better prepared 

 for learning what have been the circumstances attendant 

 on great changes of the surface, of which fresh traces 

 are seen. Although, when we were on the spot, five 

 years had elapsed since the dreadful earthquake of 

 Riobamba, by which, on the 4th of February 1797, 



