294 THE KEVOLUTIONS OF THE CRUST OF THE' EARTH. 



the point nearest to the origin of agitation is the first to be influenced 

 by the commotion which is transmitted to other points in isochronal 

 circles. 



Close observation of these isochronal circles would undoubtedly con- 

 tribute much to the knowledge of the nature of earthquakes. For this 

 end meteorological stations should be provided with instruments for 

 determining the direction and intensity of these phenomena. But all 

 the instruments for this purpose, hitherto proposed, fail either because 

 they are too complicated or on account of their want of accuracy, and 

 the seismometers employed are for the most part not as good as a bottle 

 with the inside polish taken off and filled with a liquid slightly colored. 

 Such a bottle, suitably placed in a quiet spot, will indicate, in case of a 

 commotion, by the highest points of the side wet and colored, the direc- 

 tion of the current, and by the width of the zone the relative intensity 

 for the same place. 



Many scientists distinguish in the motion of earthquakes an agitation 

 by shocks, a movement of undulation, and a movement of rotation, (or 

 of twisting.) The centers of commotion of the crust, where the effects 

 are the most violent, sometimes present the phenomenon of a shock 

 from below upward. During the great earthquake which, in 1783, de- 

 vastated Southern Calabria and the city of Messina, blocks of granite 

 were seen to jump np without changing place, wh^le men and even ani- 

 mals were thrown into the air and deposited in some cases upon much 

 higher ground. When the earthquake in 1797 destroyed the city of 

 Eeobamba, situated to the south of Quito, the corpses of several of the 

 former inhabitants were thrown upon the hill La Cullca, a height of sev- 

 eral hundred feet. It is said that during the earthquake in Chili, which 

 took place on the 7th of November," 1837, a pole sunk 8.5 meters (28 

 feet) in the ground, and sustained by iron bars, was violently thrown 

 upward, leaving a deep hole.* 



The undulatory movement of the ground is more frequent than the 

 movement bj shocks, and its relation to the latter movement is analo- 

 gous to the undulation of a surface of water compared with the spot at 

 which a body falls into it ; with the difference that the surface of the 

 soil receives the first shock from below upward. . The surface of the 

 ground is agitated as a liquid surface. Thus, to several eye-witnesses 

 of the earthquake of Lisbon, the buildings of that city appeared to 

 rise and fall like a flotilla with the waves of a heavy sea. The move- 

 ment of the ground j^roduces upon the mind an effect well described by 

 Humboldt. "This impression," says the author of the Cosmos, "does 

 not in my opinion proceed from the memory of the numerous similar 

 catastrophes recorded by history, which then crowd upon the imagina- 

 tion. What affects us is, that we lose suddenly our innate confidence 

 in the stability of the ground. From our infancy we are habituated to 

 the contrast of the mobility of the water with the immobility of the 



'' Giraid, op. vit., p. 8. 



