viii Atmospheric Phenomena 119 



strength. An animal or a human body may form part of 

 the path of least resistance and so be " struck," but this will 

 never happen if there is a better conductor near. The im- 

 pressiveness of a thunderstorm is largely due to the majestic 

 roar of the thunder, the darkness of the sky, the lurid glare 

 of the clouds, and the ominous stillness of the air ; but apart 

 from these the presence of highly electrified bodies produces 

 an indescribable effect on the nerves of many people. 

 Lightning-conductors attached to buildings serve to equalise 

 the potential of the Earth and clouds, and thus tend to prevent 

 a disruptive discharge from taking place. Thunderstorms 

 occur most frequently in the tropics, and usually during the 

 day ; in polar regions they occur very rarely, and then only 

 at night. 



174. The Aurora. In the north polar regions, where 

 thunderstorms are practically unknown, beautiful luminous 

 effects are produced at night by the Aurora borealis or 

 Northern Lights (see small map on Plate XIV.) A 

 similar appearance in the south polar regions is called 

 Aurora australis. The Aurora forms an arch or ring of 

 coloured light over the magnetic pole ( 98) at a 

 great height in the atmosphere, from 50 to 150 miles. 

 Coloured fringes and streamers shoot from this arch 

 in all directions, sometimes spreading over the whole sky, 

 and again shrinking back with a pulsing motion. The 

 Aurora appears to be caused by electrical discharges in 

 rare air, as it very closely resembles the glow seen when 

 a current traverses a "vacuum tube" containing a little 

 highly rarefied air. This theory was recently confirmed by 

 the Finnish physicist Prof. Lemstrom, who covered the top 

 of Mount Oratunturi in the north of Finland with a network 

 of wires and found a true Aurora produced when he sent a 

 current of electricity from these wires to the Earth. 3 



175. Wind. When air is heated at the Earth's surface 

 it expands, and becoming less dense, rises and flows away 

 in the upper regions of the atmosphere. The pressure of 

 the air over the region where expansion has taken place 

 thus becomes less than that of the surrounding atmosphere, 

 and air is accordingly driven in from all sides until equili- 



