DESCRIPTION OF EARTHQUAKES. 97 



a mass of fire ; and such in fact might have been the case had it 

 not been for certain disturbing forces which have fashioned the 

 mountains and valleys, and caused the dry land to be lifted up 

 above the waters. Had it not been lor these disturbing forces, 

 phenomena analogous to volcanoes and eaithquakes, the whole 

 globe would have remained under water, and man would not have 

 been called into existence. But having seen the general structure 

 of the interior of the earth, we will study the phenomena, the dis- 

 turbing forces which modify its surface, more particularly than we 

 have yet done. 



These disturbing forces are either internal or external ; first, of 

 the INFLUENCE OF INTERNAL AGENTS ON THE SURFACE OF THE 

 EARTH. 



It has been already stated (page 12) that the centre of our earth 

 is a mass of fire, to the influence of which many phenomena may 

 be referred. 



EARTHQUAKES. 



2. Description of Earthquakes. Every one has heard of the 

 terrible scourge which in a moment reduces the most nourishing 

 cities to a heap of ruins, and sometimes upturns the neighbouring 

 country. An earthquake is often preceded by rumbling, subterra- 

 neous sounds, which are frequently heard some time before thp 

 catastrophe. Tremblings more or less violent are perceived during 

 a few minutes or seconds only, which in many instances are often 

 repeated with more or less rapidity and force ; in certain cases 

 they even continue, with irregular intervals, during several days, 

 or months, or even entire years. These movements of the earth 

 are of different kinds ; sometimes they consist of jerking horizon- 

 tal oscillations, occurring at irregular intervals, sometimes of verti- 

 cal shocks, that is, in rapid and successive rising and falling of the 

 soil ; at other times of various twisting movements. Frequently 

 all the various motions take place almost at the same moment, and 

 then nothifctf can escape destruction. 



3. Sometimes an earthquake is circumscribed in narrow limits ; 

 that which happened on the 2d of February, 1828, in the island 

 of Ischia, was not felt either in the neighbouring islands or on the 

 continent. Frequently, too, it shakes an immense surface : for 

 example, the earthquake of the 17th June, 1826, in New Grenada, 

 was felt over many thousand square leagues. Sometimes it extends 

 enormous distances, as in the case of the famous earthquake of 

 Lisbon, which was felt in Lapland in one direction, and Martinique* 

 in another ; and, transversely, from Greenland to Africa, where 



2. What are earthquakes ? What is the nature of the motions produced 

 by earthquakes ? What is the duration of earthquakes ? 



3. What are the limits of earthquakes ? 



9 



