364 WRITINGS OF JOSEPH HENRY. [1855- 



electricity ; and if the upper cloud be afterward driven 

 away by the wind, the lower will be left highly negative. 



Peltier states that he can determine from the appearance 

 of a cloud whether it be positively or negatively charged. 

 Clouds negatively electrified, (according to him,) are of a 

 bluish gray color, while those which are positively charged 

 are white and exhibit at the setting sun a red appearance. 



From the foregoing considerations it must be evident that 

 in addition to the disturbance which is produced in the atmos- 

 phere by the variations of heat and moisture we must take 

 into account those that result from the changes in the electri- 

 cal condition of the atoms of moisture. Though they may 

 not be as important as the former, still they must modify the 

 conditions of the general phenomena, and no theory of 

 storms can be complete which does not include the effect 

 of this agent. 



On the principles we have developed, the discharges of 

 lightning which are exhibited in volcanic eruptions are 

 readily understood. The column of aqueous vapor, heated 

 air, and other conducting materials, which sometimes rises to 

 a great elevation from Vesuvius, must be subjected to the 

 inductive action of the earth, and consequently the elec- 

 tricity of the upper end of the column, as soon as its ele- 

 vation is sufficient to produce a condensation of the vapor, 

 by the cold of the higher regions, must send down to the 

 lower part of the column a large amount of electricity which 

 when the length is great and the ascending stream rapid, 

 will manifest itself in discharges of lightning. 



In accordance with the same principles, thunder-storms 

 have been artificially produced in a peculiar state of the 

 atmosphere. About thirty years ago a farmer at Greenbush, 

 near Albany, collected on a knoll in the middle of a field 

 a large amount of brushwood, which was set on fire simul- 

 taneously at different points, and, burning, gave rise to an 

 ascending column of heated air, extending to a great alti- 

 tude. The air rushing in to supply the upward current as- 

 sumed a rapid rotary motion, accompanied by a loud roar- 

 ing and discharges of lightning of sufficient magnitude to 



