ON ITS EXTERIOR. EARTHQUAKES. 195 



terranean thunder-clap, was heard a quarter of an hour 

 later at Truxillo, and was unaccompanied by any trembling 

 of the ground. In like manner, it was not till some time 

 after the great earthquake of New Granada, November 16, 

 1827, described by Boussingault, that subterranean detona- 

 tions, unaccompanied by any movement, were heard with 

 great regularity at intervals of thirty seconds, throughout 

 the whole Cauca Valley. The nature of the noise also dif- 

 fers greatly ; sometimes it is rolling, and occasionally like the 

 clanking of chains ; in the city of Quito it has sometimes 

 been abrupt, like thunder close at hand, and sometimes clear 

 and ringing, as if obsidian or other vitrified masses clashed, 

 or were shattered in subterranean cavities. As solid bodies 

 are excellent conductors of sound, which is propagated, for 

 example, in burnt clay with a velocity ten or twelve times 

 greater than in air, the subterranean noise may be heard at 

 great distances from the place where it has originated. In the 

 Caraccas, in the grassy plains of Calaboso, and on the banks 

 of the Bio Apure which falls into the Orinoco, there was 

 heard over a district of 2300 square (German) miles, a 

 loud noise resembling thunder, unaccompanied by any 

 shaking of the ground; whilst, at a distance of 632 miles 

 to the north east, the crater of the volcano of St. Vincent, 

 one of the small West India Islands, was pouring forth a 

 prodigious stream of lava. In point of distance, this was as 

 if an eruption of Vesuvius should be heard in the north 

 of Erance. In 1744, at the great eruption of Cotopaxi, 

 subterranean noises, as of cannon, were heard at Honda, 

 near the Magdalena River. Not only is the crater of 

 Cotopaxi about 18100 English feet higher than Honda, 

 but these two points are separated from each other by a 



