The Evolution of the Sciences 



globe reach their melting temperature at a depth 

 of between forty and sixty -five miles, which 

 would also be the mean thickness of the solid 

 crust of the globe. This hypothesis is sup- 

 ported by observations of another order. 



Those who believe the form of the earth to 

 be invariable are strangely mistaken. Their 

 mistake is not shared by the inhabitants of 

 some central American provinces, who have 

 never seen a year go by without the earth 

 shaking under their feet. 



In our own countries the vibrations, though 

 weaker, are not less frequent. M. Fuchs 

 counted 1184 earthquakes between 1865 and 

 1873, and a Scientific Committee registered 166 

 in Switzerland during the single year 1881. In 

 addition to the violent shocks which force them- 

 selves on our attention by their disastrous con- 

 sequences slighter tremors are continually run- 

 ning through the ground. They are recorded in 

 numerous observatories by instruments called 

 seismographs. The simplest consists of a pendu- 

 lum, free to oscillate in every direction, and 

 provided at its lower extremity with a marker. 



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