156 DISCOVERY CH. 



in a thick cloud, which, by reason of its lightness, 

 breaketh forth violently, the rupture of the cloud 

 maketh a crack, and the divulsion by reason of the 

 blackness causeth a flashing light." 



This represents an early attempt to account for the 

 phenomena on a naturalistic basis, instead of referring 

 them to Jove and his thunderbolts. Clouds clashing 

 together instead of cracking seem to the child to provide 

 sufficient reason for the cause of thunder a view 

 expressed by Milton in the lines : 



As when two black clouds, 



With heaven's artillery fraught, come rattling on 

 Over the Caspian ; then stand, front to front 

 Hovering a while, till winds the signal blow 

 To join their dark encounter in mid-air. 



Later views were that lightning and thunder were 

 caused by the explosion of gases in the air, or by the 

 spontaneous combustion of particles of "a subtle 

 sulphur." No means existed for the production of long 

 " electric sparks " and, therefore, no suggestion could 

 be made of any relation between such discharges and 

 those taking place in the clouds. Early experimenters 

 with electrical machines did not overlook the resem- 

 blance between the flash and crack of their electric 

 sparks and the similar effects observed during thunder- 

 storms. Benjamin Franklin, in 1749, stated the points 

 of similarity between the electric discharges and 

 lightning to be : (1) Giving light, (2) Colour of the light, 

 (3) Crooked direction, (4) Swift motion, (5) Being con- 

 ducted by metals, (6) Crack or noise in exploding, (7) 

 Subsisting in water or ice, (8) Rending bodies it passes 

 through, (9) Destroying animals, (10) Melting metals, 

 (11) Firing inflammable substances, (12) Sulphurous 

 smell. 



