Curiosities of Science. 



on the surface of the cylinder ; but as soon as it oscillates, this 

 line becomes interrupted and irregular, and these irregularities 

 indicate the time of the commencement of an earthquake, to- 

 gether with its duration and intensity.* 



Elastic fluids are doubtless the cause of the slight and per- 

 fectly harmless trembling of the earth's surface, which has often 

 continued for several days. The focus of this destructive agent, 

 the seat of the moving force, lies far below the earth's surface ; 

 but we know as little of the extent of this depth as we know of 

 the chemical nature of these vapours that are so highly com- 

 pressed. At the edges of two craters, Vesuvius and the tower- 

 ing rock which projects beyond the great abyss of Pichincha, 

 near Quito, Humboldt has felt periodic and very regular shocks 

 of earthquakes, on each occasion from twenty to thirty seconds 

 before the burning scoriae or gases were erupted. The intensity 

 of the shocks was increased in proportion to the time interven- 

 ing between them, and consequently to the length of time in 

 which the vapours were accumulating. This simple fact, which 

 has been attested by the evidence of so many travellers, furnishes 

 us with a general solution of the phenomenon, in showing that 

 active volcanoes are to be considered as safety-valves for the 

 immediate neighbourhood. There are instances in which the 

 earth has been shaken for many successive days in the chain of 

 the Andes, in South America. In certain districts, the inha- 

 bitants take no more notice of the number of earthquakes than 

 we in Europe take of showers of rain ; yet in such a district 

 Bonpland and Humboldt were compelled to dismount, from the 

 restiveness of their mules, because the earth shook in a forest 

 for fifteen to eighteen minutes without intermission. 



EARTHQUAKES AND THE MOON. 



From a careful discussion of several thousand earthquakes 

 which have been recorded between 1801 and 1850, and a com- 

 parison of the periods at which they occurred with the position 

 of the moon in relation to the earth, M. Perry, of Dijon, infers 

 that earthquakes may possibly be the result of attraction ex- 

 erted by that body on the supposed fluid centre of our globe, 

 somewhat similar to that which she exercises on the waters of 

 the ocean ; and the Committee of the Institute of France have 

 reported favourably upon this theory. 



THE GREAT EARTHQUAKE OF LISBON. 



The eloquent Humboldt remarks, that the activity of an ig- 



* Mr. R. Mallet, F.R.S., and his son Dr. Mallet, have constructed a seismo- 

 graphic map of the world, with seismic bands in their position and relative 

 intensity; and small black discs to denote volcanoes, femaroles, and soltataras, 

 and shades indicating the areas of subsidence. 



