LIGHTNING AND THUNDER. 223 



the earth, or from the earth to the cloud ; as in either 

 case, the discharge will follow the path of least resist- 

 ance ; which is always through the conductor, when 

 properly constructed. 



HEAT LIGHTNING. The phenomenon, known as 

 heat lightning, is probably nothing more than the or- 

 dinary electric discharge from clouds invisible to the 

 observer, and so distant that the thunder is inaudible. 

 Such lightning is generally observed at night, near the 

 horizon ; and close observation will show, either the 

 existence of clouds, indistinctly visible in the darkness, 

 or the probability of the discharge occurring from 

 clouds below the horizon. 



Its existence, independent of clouds, is claimed from 

 the fact, that it has been observed when no thunder 

 storm had occurred within a radius of one hundred 

 miles. But, not only lightning, but clouds are often 

 visible at greater distances. On the level surface 

 round Chicago, the author has frequently observed 

 heavy thunder storms, eighty miles distant, as shown 

 by subsequent reports, when both clouds and lightning 

 were distinctly visible, though the thunder was not 

 audible. 



TORNADOES. As an electric origin has been claimed 

 for tornadoes, it is proper to remark, in conclusion, 

 that recent investigation has demonstrated that they 

 are chiefly due to currents of air, generated by differ- 

 ences of atmospheric temperature and pressure, and 

 modified by other causes: and while electricity may 

 intensify their force, it cannot be considered as their 

 primary cause. 



