208 COSMOS. 



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During- the long-continued trembling of the ground in the 

 Piedmontese valleys of Pelis and Clusson, the greatest changes 

 in the electric tension of the atmosphere were observed while 

 the sky was cloudless. The intensity of the hollow noise which 

 generally accompanies an earthquake does not increase in the 

 same degree as the force of the oscillations. I have ascertain- 

 ed with certainty that the great shock of the earthquake of 

 Riobamba (4th Feb., 1797) — one of the most fearful phenom- 

 ena recorded in the physical history of our planet — was not 

 accompanied by any noise whatever. The tremendous noise 

 {el gran ruido) which was heard below the soil of the cities 

 of Quito and Ibarra, but not at Tacunga and Hambato, near- 

 er the center of the motion, occurred between eighteen and 

 twenty minutes after the actual catastrophe. In the cele- 

 brated earthquake of Lima and Callao (28th of October, 

 1746), a noise resembling a subterranean thunder-clap was 

 heard at Truxillo a quarter of an hour after the shock, and 

 unaccompanied by any trembling of the ground. In like 

 manner, long after the great earthquake in New Granada, on 

 the 16th of November, 1827, described by Boussingault, sub- 

 terranean detonations were heard in the whole valley of Cauca 

 during twenty or thirty seconds, unattended by motion. The 

 nature of the noise varies also very much, being either rolling, 

 or rustling, or clanking like chains when moved, or like near 

 thunder, as, for instance, in the city of Quito ; or, lastly, clear 

 and ringing, as if obsidian or some other vitrified masses were 

 struck in subterranean cavities. As solid bodies are excellent 

 conductors of sound, which is propagated in burned clay, for 

 instance, ten or twelve times quicker than in the air, the sub- 

 terranean noise may be heard at a great distance from the 

 place where it has originated. In Caraccas, in the grassy 

 plains of Calabozo, and on the banks of the Rio Apure, which 

 falls into the Orinoco, a tremendously loud noise, resembling 

 thunder, was heard, unaccompanied by an earthquake, over 

 a district of land 9200 square miles in extent, on the 30th of 

 April, 1812, while at a distance of 632 miles to the north- 

 east, the volcano of St. Vincent, in the small Antilles, poured 

 forth a copious stream of lava. With respect to distance, this 

 was as if an eruption of Vesuvius had been heard in the north 

 of France. In the year 1744, on the great eruption of the 

 volcano of Cotopaxi, subterranean noises, resembling the dis- 

 charge oi' cannon, were heard in Honda, on the Magdalena 

 River. The crater of Cotopaxi lies not only 18,000 feet high- 

 er than. Honda, but these two points are separated by the co- 



