78 nature's flMracles. 



a state of strain like a bent bow or a wound- 

 up spring. When these moisture globules con- 

 dense into drops of water the potential energy 

 is set free and becomes active either as heat or 

 electricity. The cloud gathers up the energy 

 into a condensed form, and when the tension 

 gets too great a discharge takes place between 

 the cloud and the earth or from one cloud to 

 another, which to an extent equalizes the 

 energy. 



In most cases of thunder and lightning it 

 is only a discharge from cloud to cloud un- 

 equally charged. This does not relieve the 

 tension between the earth and the cloud, but 

 distributes it over a larger area. The reason 

 for this constant electrical difference between 

 the earth and the upper regions of atmosphere 

 is not well understood, except that primarily it 

 is an effect of the sun's rays. Evaporation 

 may and probably does play a part, and tin- 

 same causes that give rise to the auroral dis- 

 play may contribute in some way to the same 

 result. Evaporation does not always take 

 place at the earth's surface. Cloud formations 

 may be evaporated in the upper air into in- 

 visible moisture spherules, and charged at the 

 time with potential energy. If we go up into 

 a high mountain when the conditions are right, 

 we can witness the effect of this condition of 

 electrical charge or strain between the upper 

 regions of the atmosphere and the earth, and 



