Earthquakes are generally attended with sounds, sometimes like the howling of a storm, or the rumbling of subterranean thunder, at others like the clashing of iron chains, or as if a number of heavily laden waggons were rolling rapidly over the pavement, or as if enormous masses of glass were suddenly shivered to pieces. As solid bodies are excellent conductors of sound (burnt clay, for instance, propagating it ten or twelve times more rapidly than the air), the subterranean noise may be heard at a vast distance from the primary seat of the earthquake. In Caraccas, in the grass-plains of Calabozo, and on the banks of the Rio Apure, which falls into the Orinoco, a dreadful thunder-like sound was everywhere heard on April 30, 1812, without any simultaneous trembling of the earth, at the time when, at the distance of 158 geographical miles, the volcano of St. Vincent, in the Lesser Antilles, was pouring out of its crater a mighty lava-stream. This was, according to distance, as if an eruption of Vesuvius were heard in the north of France. In the year 1744, during the great eruption of the volcano Cotopaxi, a subterranean noise like the firing of cannon was heard at Honda on the Magdalena river. But the crater of Cotopaxi is 17,000 feet higher than Honda, and both points, situated at a distance of 109 geographical miles, are moreover separated by the colossal mountain masses of Quito, Pasto, and Popayan, and by numberless ravines and deep valleys. The sound was certainly transmitted, not through the air, but through the earth, and must have proceeded from a very considerable depth. But noise is not the necessary attendant of an earthquake, for many instances are known in which the most violent shocks have been completely noiseless. No subterranean sounds were heard during the terrific earthquake which destroyed Riobamba on February 4, 1797, and the same circumstance is mentioned in the narratives of many of the Chilian earthquakes. The phenomenon of sound, when unaccompanied by any perceptible vibration, makes a peculiarly deep impression on the mind, even of those who have long inhabited a country subject to frequent earthquakes. They tremble at the idea of the catastrophe which may follow. A remarkable instance of a long protracted noise without any trembling of the earth occurred in 1784, at the wealthy mining town of Guanaxuato in Mexico, where the rolling of subterranean thunder, with now and then a louder crash, was heard for more than a month, without the slightest shock, either on the surface of the earth, or in the neighbouring silver mines, which are 1,500 feet deep. The noise was confined to a small space, so that a few miles from the town it was no longer audible. Never before had this phenomenon been known to occur in the Mexican highlands, nor has it been repeated since.