some time, a vibratory motion will be 

 produced at the surface of the earth. If 

 these alternate dilatations and compres- 

 sions should succeed one another at very 

 small intervals, they would excite a like 

 motion in the air and thereby occasion 

 a considerable noise. The noise that is 

 usually observed to precede or accompany 

 earthquakes, is probably owing partly to 

 this cause, and partly to the grating of 

 the parts of the earth together, occasioned 

 by that wave-like motion before men- 

 tioned " (Art. 6)\ 



" As a small quantity of vapour almost 

 instantly generated at some considerable 

 depth below the surface will produce a 

 vibratory motion, so a very large quantity 

 (whether it be generated almost instantly, 

 or in any small portion of time) will pro- 

 duce a wave-like motion. The manner 

 in which this wave-like motion will be 

 propagated may, in some measure, be 

 represented by the following experiment. 

 Suppose a large cloth or carpet (spread 

 30 



