THE AURORA BOREALIS 219 



The relative rates of diffusion are inversely as the 

 square roots of the densities, and are given in the second 

 column. For example, oxygen escapes into a neighbour- 

 ing layer twice as quickly as xenon ; and helium nearly 

 three times as fast as oxygen. Now it is evident that the 

 gases which will escape most slowly are krypton and 

 xenon; carbon dioxide and argon come next in order; 

 while nitrogen, neon, water vapour, and helium escape 

 more rapidly in the order given. If then a jar with 

 porous walls were full of air, and were exposed to some 

 indifferent atmosphere, the gas remaining in the jar after 

 some time would contain more of the heavier and less of 

 thelighter gases proportionally to the original amounts 

 present. 



The third premiss in the argument is that in equatorial 

 regions there is an upward current of air, due to the 

 warming of the earth by the nearly vertical rays of the 

 sun and the consequent expansion of the air in contact 

 with the soil or the sea ; while in the polar regions there 

 is a continual downward current, produced by the cooling 

 of the air in contact with the ice of the polar caps. This 

 circulation of the atmosphere was investigated by Professor 

 James Thomson in 1857, and his Bakerian lecture on the 

 subject appeared in the Philosophical Transactions for 

 1892, p. 653. The conclusion to which he came is that 

 the upward atmospheric current at the equator on reaching 

 the higher regions of the atmosphere, divides into two, 

 and while one part of the air travels in a north-easterly 

 direction, the other half travels in a south-easterly direction 

 towards the north and south poles respectively. Arrived 

 at the neighbourhood of the polar caps, the air descends 

 and, broadly stated, travels back near the surface of the 

 earth again towards the equator. We need not here regard 

 eddies which occur near the tropics of Cancer and Capri- 

 corn ; the main features are sufficient for our purpose. 



