xni EFFECTS OF TEMPERATURE 241 



plain to you from the following : In the winter 3 

 season, when there is cold on the earth s surface, 

 the wells are warm, and caves and all underground 

 retreats equally so. The heat, yielding possession 

 of the upper regions to the cold, retreats down 

 there. When it reaches the lower regions, and 

 is accumulated there to the utmost, the denser it 

 is, the more powerful is it. To this a further 

 supply is added, to which what has already 

 gathered, and is compressed into a narrow space, 

 of necessity gives way. The same thing happens 

 from the opposite cause when a greater quantity of 

 cold is borne down to these recesses. All the heat 4 

 that lurks there gives way to the cold, and retires 

 to the narrow passages, and is driven onward with 

 great impetuosity. The nature of the two, as I 

 have said, does not allow agreement, or abode in 

 the same place. In its flight, then, and eager haste 

 to escape at all hazards the air pushes back and 

 tosses about all that lies near it. This is why, 

 previous to an earthquake, a roaring is usually 

 heard, through the tumult of the winds in the 

 earth s bowels. For not otherwise, as our poet 5 

 Virgil says, could 



The earth bellow beneath our feet and the lofty peaks be moved, 



were not this the work of the winds. In this 

 contest again there are ups and downs. There are 

 cessations in the massing of the heat and, in turn, 

 in its emission. Then the cold, too, is restrained 

 and gives way, but some day soon it will be more 

 powerful again. While, therefore, the alternating 

 forces rush to and fro, and the air moves hither 

 and thither, the earth is shaken. 



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