98 PHYSICAL SCIENCE BK. n 



causes of things and noted their effects. She per- 

 formed a service far more valuable than the 

 inspection of lightning in thus comparing results 

 with the principles from which they are derived. 



LIV 



1 I WILL at this point revert to Posidonius' opinion 

 of the cause of thunder. From the earth and its 

 confines are exhaled certain elements, partly moist, 

 partly dry and smoke-like. The latter element 

 remains in the sky as material for lightning, while 

 the former falls in rain. The dry smoky particles 

 that reach the atmosphere will not allow themselves 

 to be enclosed in clouds, but burst their envelope. 

 Thence comes the report which we name thunder. 

 Besides this, anything in the atmosphere itself that 

 is rarefied is at the same time dried and heated up. 



2 This also, if it is enclosed, seeks an exit with equal 

 eagerness, and causes a report as it escapes. On 

 one occasion it makes a complete burst, and the 

 thunder is consequently the more violent ; on 

 another it escapes by degrees in small portions. 

 Air of this kind, then, by either bursting or flying 

 through the clouds, produces peals of thunder. The 

 rolling of the air enclosed in a cloud is the most 

 potent cause of setting fire to what is struck. 



LV 



i THUNDER is, in short, simply the report of ex- 

 plosions of dry air, which cannot occur unless there 

 is either friction or a rent in a cloud. Posidonius 



