ON ITS EXTERIOR. EARTHQUAKES. 179 



of the 29th of July 1846. The velocity of propagation 

 obtained for that earthquake was 1466 English feet in 

 a second. This is a rapidity exceeding that of the -wave 

 of sound in the air ; but if we consider that the velo- 

 city of the sound-wave in water is, according to Colladon 

 and Sturm, 5016 English feet, and in cast-iron tubes, 

 according to Biot, 11,393 feet in a second, the result 

 found for the earthquake-wave will appear to us very 

 small. For the Lisbon earthquake of Nov. 1, 1755, 

 from the coast of Portugal to that of Holstein, Schmidt 

 found (from less accurate data) a velocity more than 

 five times greater than in the case of the Ehine earth- 

 quake of the 29th of July 1846. Between Lisbon and 

 Grluckstadt (a distance of 1180 English geographical 

 miles), the rate derived by him is 7955 English feet in 

 a second ; which is still 3438 feet less than takes place 

 in cast iron. ( 251 ) 



Earthquakes, and sudden fiery eruptions of volcanoes, 

 recurring after long intervals of repose, whether they 

 send forth merely scoriae, or, like intermitting water- 

 springs, pour forth fluid molten earths in lava streams, 

 have indeed the same causal connection, or common 

 origin, in the high temperature of the interior of our 

 planet ; but one of these phenomena shows itself in most 

 cases quite independently of the other. For example, 

 violent earthquakes, in their linear extension along the 

 chain of the Andes, agitate regions in which there are 

 volcanoes which are not extinct, and are indeed often in 

 a state of activity, yet without causing in them any per- 

 ceptible excitement. In the great catastrophe of Eio- 

 bamba, the neighbouring volcano of Tunguragua and the 

 rather more distant one of Cotopaxi remained quite tran- 



