EFFECT OF UNDULATIONS. 



depressed through a very considerable height or depth, without 

 producing any superficial derangement, but if such elevation and 

 depression take place with much rapidity, great disturbance 

 must ensue. 



15. Humboldt, who has been personal witness of a considerable 

 number of these phenomena, and has elaborately investigated the 

 recorded effects of the most remarkable of them, says that the 

 undulations are propagated chiefly in parallel lines, and with a 

 progressive velocity of from twenty to thirty miles per minute. 

 He observes that the cases in which the waves issue from a 

 centre of undulation, and are propagated in circles around it, are 

 more rare, and that when it takes place, the height of the waves 

 diminish as their distance from the common centre increases. 



In general, the undulations are inconsiderable in their vertical 

 height and velocity of oscillation, so that in places affected by 

 them, the strength of buildings is sufficient to resist their effects, and 

 we constantly hear of slight shocks of earthquakes being sensible, 

 which are attended with no injurious consequences. Bells are 

 sometimes thus rung, and furniture and other loose objects more 

 or less displaced without other more serious consequences. 



16. The vertical shock however, in places more subject to these 

 visitations, is sometimes attended with far more grave effects. In 

 the case of the earthquake, by which the town of Eiobamba, at 

 the foot of Chimborazo, was destroyed in 1797, the bodies of many 

 of the inhabitants were hurled to a height of several hundred 

 feet, and thrown upon the hill of La Cullca, beyond the small 

 river Lican. 



17. Humboldt cites examples of the circular propagation of 

 terrestrial undulations at the Holy Sea, or Lake Baikal, in 

 Siberia (between lat. 51 and 55, and long. 103 and 110 E.), 

 and in the Celestial Mountains, or Thian-schan, in Chinese 

 Turkestan, (between lat. 42 and 43, and long. 80 and 90 E.) 

 The intermediate region is subject to the double influence of the 

 circular undulations propagated from these two centres, and 

 Humboldt ingeniously explains the freedom from earthquakes of 

 a certain intermediate tract between two such centres of undula- 

 tion, by the principle of interference which plays so important a 

 part in optics and acoustics. He supposes that along such a 

 tract the crests of the waves of one system coincide with the 

 hollows of those of the other, so that the ground being just as 

 much elevated by the one as it is depressed by the other, remains 

 undisturbed, while the country at either side is agitated, and 

 perhaps devastated, by the shocks. 



18. In certain cases, the motion imparted to the surface is not 

 merely that of undulation properly so called, which, as already 



153 



