212 ELEMENTS OF STATIC ELECTRICITY. 



adequate, when we consider the results which must fol- 

 low from the rush of the enormous energy of a thunder 

 cloud, along a line, perhaps five miles in length, in an 

 infinitesimal fraction of a second. 



And here, as in the case of the spark, it is quite un- 

 necessary to suppose the passage of any material sub- 

 stance through the air, producing partial vacuum and 

 collapse, or the occurrence of anything in the nature of 

 an explosion, producing similar results. It is more in 

 accordance with the known laws of electric movement, 

 to suppose that the electric energy has used the air as 

 the medium in which to travel; and thus produced the 

 vibratory motion. 



Common observation shows, that in explosions where 

 the expenditure of energy must often be far less than 

 in the electric discharge between clouds, the vacuum 

 and collapse shatter window-glass in the vicinity; while 

 the heaviest thunder produces only a slight tremor in 

 adjacent buildings ; proving that such vacuum and col- 

 lapse cannot result from an electric discharge. 



The succession of reports accompanied by a continu- 

 ous rumble, heard so frequently during a thunder storm, 

 has been considered, by some observers, as a series of 

 echoes from a single report; and by others, as a num- 

 ber of separate reports, from discharges occurring si- 

 multaneous]} T , at different distances from the observer, 

 and heard in the order of their distance. 



An echo requires the intervention of an extended 

 surface, as a wall or its equivalent; and observation 

 shows, that the under surface of a dense thunder cloud 

 is of this character, being remarkably uniform, though 

 its upper surface may be quite the reverse : and it is also 

 evident, that this under surface, resting on tho denser 



