206 PHYSICAL SCIENCE BK. v 



. . . hung over dark retreats. 



In some places it contains voids that have no 

 moisture. Though there is no light there to 

 show the distinctions in the air, yet I venture to 



2 assert that cloud and mist settle in that gloom. 

 Above ground cloud and mist surely do not 

 exist because they are seen ; but, rather, they 

 are seen because they exist. Well, there too rivers 

 none the less exist that they are not seen. You 

 must understand that down there rivers flow equal 

 in size to our own. Some glide gently, others 

 resound as they tumble down headlong over the 

 broken ground. So must not you equally allow 

 that there are some lakes underground and some 



3 water in pools without an exit ? This being so, it 

 is of necessity that the air be charged with moisture, 

 and that, being charged, it lean in one direction, rais- 

 ing the wind by its propulsion. We must recognise, 

 therefore, that from those subterranean clouds blasts 

 of wind are raised in the dark, what time they have 

 gathered strength sufficient to remove the obstacles 

 presented by the earth, or can seize upon some open 

 path for their exit, and from this cavernous retreat 



4 can escape toward the abodes of men. Now it is 

 obvious that underground there are large quantities 

 of sulphur and other substances no less inflammable. 

 When the air in search of a path of escape works its 

 tortuous way through ground of this nature, it 

 necessarily kindles fire by the mere friction. By 

 and by, as the flames spread more widely, any 

 sluggish air there may be is also rarefied and set in 

 motion ; a way of escape is sought with great 

 roaring and violence. This point I will elaborate 

 in more detail when I go on to treat of earthquakes. 



