194 BEACTION OF THE INTERIOR OF THE EARTH 



4th of November, 1799, I found no change either in 

 the magnetic declination or intensity from a strong shock 

 of earthquake; but, on that occasion, I observed with 

 astonishment that the inclination was diminished 48'. ( 186 ) 

 I had no reason to suspect any error, although, during 

 a great number of other shocks and earthquakes ex- 

 perienced by me in the highlands of Quito and Lima, 

 the inclination, as well as the other elements of terres- 

 trial magnetism, always remained unaltered. If, however, 

 these deep-seated terrestrial movements are not generally 

 announced by any peculiar state of the atmosphere or 

 appearance of the sky, it is, on the other hand, as we shall 

 soon see, not improbable, that in some very violent earth- 

 quakes the aerial strata have participated, and that the 

 phenomena are not, therefore, always purely dynamic. 

 During the long-continued trembling of the ground in the 

 Piedmontese valleys of Pelis and Clusson, great variations in 

 the electric tension of the atmosphere were remarked, quite 

 independently of any storm, and when the sky was perfectly 

 clear. 



The hollow noise which most frequently accompanies 

 earthquakes by no means increases in proportion to the 

 violence of the oscillations. I- have distinctly ascertained 

 that the great shock of the earthquake of Eiobamba (4th 

 February, 1797), one of the most terrible phenomena in the 

 physical history of our globe, was unaccompanied by any 

 noise; the great subterranean detonation (el gran ruido), 

 which was heard at the cities of Quito and Ibarra, (but not 

 at Tacunga and Hambato which were nearer the center of 

 the movement,) occurred eighteen or twenty minutes after 

 the catastrophe. In the celebrated earthquake of Lima and 

 Callao, October 23th 1746, a noise, resembling a sub- 



