132 RAMBLES OF A NATURALIST. 



lations like this, the pressure of an atmosphere on a 

 surface whose extent we know, may be regarded as 

 equal to the weight of a column of water of the 

 same base, and of eleven yards in height. Conse- 

 quently the total effect produced by the machines of 

 Saint-Germain may be represented by a weight O A 

 about 150 tons. 



A column of water raised from the level of the 

 sea to the summit of Etna, would exert a pressure of 

 300 atmospheres, but the fluid lava is very nearly 

 three times heavier than water. Consequently when 

 this lava flows over the margin of the terminal cone, 

 its pressure at the level of the plain will equal a 

 force of 900 atmospheres, while its force at the 

 bottom of the crater itself will be equal to the 

 pressure of 1,800 atmospheres. The weight of this 

 pressure on every square yard of surface will there- 

 fore be equal to more than 40,000 tons. 



Now we know that the pressure of liquids is 

 exerted in all directions at once. Consequently each 

 square yard of the vault which supports the volcano 

 is subjected to a force, acting from below upwards, 

 which is 283 times greater than that of the machines 

 of Saint-Germain. In the crater alone, the total 

 force which is solely employed in sustaining the 

 column of lava at the level of the orifice is equal to 

 53,262,500 times that of these machines. This is a 

 force of more than twenty-one thousands of millions 

 of horses. 



Hitherto we have supposed that the steam engine 

 was in perfect working order, and that the lava rose 

 easily to the margin of the crater. We know, 



