7 o PHYSICAL SCIENCE BK. n 



may say that the bolt is a flash that has been 

 conveyed right down to the ground. It is not for 

 the purpose of refinement of terms that I deal at 

 some length with them, but in order to prove the 

 phenomena related and of the same category and 

 character. A bolt is something more than a flash. 

 Inverting the statement, a flash is all but a bolt. 



XXII 



1 Now that it is agreed that the two things are both 

 fire, let us see how fire arises on earth, for no doubt 

 the same method prevails aloft. There are two 

 common methods of producing fire one by striking 

 it out, as, for example, from a stone ; the other by 

 the more tedious method of friction, as when two 

 pieces of wood are rubbed together for some time. 

 It is, of course, not every kind of substance that 

 gives the desired result ; you must choose one 

 suitable for giving out fire, for example, laurel, ivy, 

 and other trees familiar to shepherds for this 

 purpose. Probably, therefore, clouds may in the 

 same way emit fire either from a blow or from 



2 friction. Consider for a moment the force with which 

 squalls rush forth, the impetuous eddying revolu- 

 tion of the whirlwind. Anything that encounters 

 a missile from an engine of war is scattered and 

 removed and driven far from its position. What 

 wonder, then, that such violence in the wind extracts 

 fire either from some external object or merely from 

 itself? You can readily see what a glow all 

 neighbouring bodies grazed by its passage must 

 receive. But the force of storms cannot for a 

 moment be compared with the energy of the 



