114 Proceedings of the Royal Physical Society. 



whole aggregate effect of volcanoes, when compared with 

 the supposed vast magnitude of a central heat, sinks into 

 perfect insignificance. 



II. Earthquake Phenomena. — The phenomena of earth- 

 quakes appear to have an intimate connection with those of 

 subterranean causes, whatever they may be, from which vol- 

 canoes originate, so that it is scarcely possible to separate 

 the two actions from an identity of cause. Although earth- 

 quakes are found to occur most frequently in volcanic coun- 

 tries, they occasionally happen in localities not subject to 

 volcanic action, and in this case it has been generally ob- 

 served that the shocks had a greater intensity. It has also 

 been noticed that the most severe shocks are usually of 

 shortest duration. The terrific earthquake which occurred 

 at Lisbon on the 1st November 1755, lasted only six minutes, 

 and in that brief space of time occasioned the destruction of 

 60,000 persons. The line of devastation extended from 

 Lisbon north to Oporto and south to the bay of Cadiz, a dis- 

 tance of 300 miles. The area of concussion formed an im- 

 mense ellipse, the longest diameter of which extended from 

 Abo in Finland south to the Canary Islands, a linear space 

 of 2700 miles. The area of vibration through which the 

 waters were observed to oscillate from the shock, included 

 a superficies four times that of Europe, affecting the lakes in 

 Scotland, those in the north of Germany, the springs in 

 Hungary, the Atlantic Ocean across to the West Indies, and 

 even some of the lakes in Canada. Three movements have 

 been observed to take place during an earthquake — a ver- 

 tical, a horizontal, and a rotatory. The vertical and hori- 

 zontal often appear to take place together ; the rotatory 

 movement is the least frequent, but always the most dan- 

 gerous. The horizontal movements are propagated in 

 undulations like waves in the sea. They accompany every 

 great earthquake, and travel at the rate of 20 to 30 miles a 

 minute. The three movements were remarkably illustrated 

 by the tremendous earthquake which destroyed the town of 

 Eiobamba, in the province of Quito, in South America, on 

 February 4, 1797, whereby 40,000 inhabitants perished. 



