xix SUBTERRANEAN VOIDS 249 



subterranean disturbances to the voice of a person 

 who puts his head into a barrel and begins to sing 

 out. In that case there is a kind of quavering 

 as the voice extends and resounds through the 

 whole hollow space ; slight as the movement is, it 

 passes all round the vessel in which it is enclosed, 

 grazing its sides and causing disturbance all through. 

 In the same way the vast empty caverns that 

 stretch down beneath the earth have atmosphere 

 of their own, on which other air coming from above 

 falls with violence. The agitation produced differs 

 in no wise from that of the empty vessels which I 

 have just mentioned, when they resound through 

 shouting into them. 



XX 



LET us now go on to consider the authors who i 

 have alleged as causes all the different factors 

 mentioned, or, at any rate, several of them. 

 Democritus is one of those who think that several 

 are concerned. He asserts that the earthquake is 

 produced sometimes by air, sometimes by water, 

 sometimes by both. He pursues the argument 

 in the following way : Some portion of the earth 

 is hollow, in which a large quantity of water has 

 gathered. Part of this water is thinner and less 

 dense than the rest. When it is driven back by 

 a heavy mass descending upon it from above, it 

 comes violently against the earth, causing a com- 

 motion of it. The fluctuating movement of the 

 water cannot take place without corresponding 

 movement of the body on which it impinges. 

 Besides, what we said a little above regarding air 2 



