258 WORLD. 



There is something startling in the idea tha,t our earth, or rath- 

 er its crust, is perhaps but a few hundred miles in thickness, or 

 in other words, that our globe is a hollow ball of no very great 

 dimensions. It is a well established proposition that, under influ- 

 ence of the attraction of gravitation, a body, or a mass of matter, 

 placed any where within a* hollow globe, as at a or b, (see the^ dia- 

 gram below), will remain at rest wherever it may be situated. 



Hence, whether the interior of the hollow globe be molten or 

 not, the mass will not be displaced, or in other words, it will have 

 no tendency to move, unless operated upon by other force than 

 the attraction of gravitation. 



The name earthquake has been given to those convulsions of 

 supposed igneous origin, which cause the surface of the earth to 

 heave, or undulate, producing rents, and generally precursing the 

 eruption of some volcano, The region of violent earthquakes, 

 is generally the site of some active volcano, and the paroxysms 

 of an earthquake, are generally relieved by a volcanic eruption, 

 Thus, during the earthquake which overturned Lima in 1746, and 

 which was one of the most terrible which has been recorded, four 

 volcanoes opened in one night, and the agitation, of the earth 

 ceased. The phenomena attending earthquakes are various, 

 sometimes there is but a slight undulatory movement, barely suf- 

 ficient to cause the lighter articles upon the surface to change 

 places. Persons unacquainted with* the phenomena of earth- 

 quakes, suppose themselves seized with a sudden giddiness. Of- 

 ten the first shocks are of this light character, then gradually be- 



