nS The Realm of Nature CHAP. 



small water globules which combine to form it. A heavy 

 shower of rain rapidly carries off the electricity, reducing the 

 potential of a cloud to that of the Earth. In certain states 

 of the atmosphere which are not yet thoroughly understood, 

 silent electric discharge takes place between pointed bodies, 

 such as flagstaff's or the masts of ships, and the air. This 

 is accompanied by a pale brush-shaped light, which goes 

 by the name of St. Elmo's fire. Air which is almost free 

 from water-vapour is a nearly perfect non-conductor ( 77), 

 and in the dry climates of mountain observatories and high 

 latitudes in winter, electricity produced by friction is not 

 immediately conducted away to the Earth as it is in damp 

 air. In Canada one can often light a gas-jet by an electric 

 spark from the finger, produced by shuffling the feet on the 

 carpet ; and at Pike's Peak observatory in the United States 

 the friction of opening a drawer or shutting a door often 

 gave rise to electricity enough to give a severe shock. 



173. Lightning and Thunder. When the electric 

 potential of a cloud becomes much higher than that of the 

 Earth or another cloud, a disruptive discharge takes place 

 between them through the air ( 78). The .electrical 

 energy is mainly converted into heat by the resistance of 

 the~aif, the particles of which become instantaneously white 

 hot ; but the passage of the electric current is so rapid that 

 only a brilliant flash is visible. The intensely heated air 

 expands suddenly, and then as suddenly contracts, setting 

 up a succession of air waves ( 58) all along the line of the 

 flash. These reach the ear as a prolonged growl or roar, 

 or as a sharp rattling explosion, according to the distance 

 of the observer and to the direction of the flash. The 

 sound is prolonged by echoes from the Earth's surface and 

 hills, or from clouds. The electric discharge follows the 

 path of least resistance, and as vegetable juices offer less 

 resistance to it than air, trees are often traversed by the 

 current. The sap between the wood and the bark is so 

 heated by the discharge that steam is formed with explosive 

 violence, splitting off the bark, tearing away branches, and 

 ploughing deep furrows in the solid wood, as if the tree 

 had been struck by a solid spear hurled with gigantic 



