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ISLAND LIFE 



vapour supplied for condensation was also exceptionally- 

 great. The greatest quantity of snow falls in the Arctic 

 regions in summer and autumn, and with us the greatest 

 quantity of rain falls in the autumnal months. It seems 

 probable, then, that in all northern lands glaciation would 

 commence when autumn occurred in aphelion. All the 

 rain which falls on our mountains at that season would 

 then fall as snow, and, being further increased by the snow 

 of winter, would form accumulations which the summer 

 might not be able to melt. As time went on, and the 

 aphelion occurred in winter, the perennial snow on the 

 mountains would have accumulated to such an extent as 

 to chill the spring and summer vapours, so that they too 

 would fall as snow, and thus increase the amount of de- 

 position ; but it is probable that this would never in our 

 latitudes have been sufficient to produce glaciation, were 

 it not for a series of climatal reactions which tend still 

 further to increase the production of snow. 



Action of IldeoTological Causes in intensifying Glaciation. 

 — The trade-winds owe their existence to the great differ- 

 ence between the temperature of the equator and the 

 poles, which causes a constant flow of air towards the 

 equator. The strength of this flow depends on the differ- 

 ence of temperature and the extent of the cooled and 

 heated masses of air, and this effect is now greatest be- 

 tween the south pole and the equator, owing to the much 

 greater accumulation of ice in the Antarctic regions. The 

 consequence is, that the south-east trades are stronger than 

 the north-east, the neutral zone or belt of calms between 

 them not being on the equator but several degrees to the 

 north of it. But just in proportion to the strength of the 

 trade-winds is the strength of the anti-trades, that is, the 

 upper return current which carries the warm moisture- 

 laden air of the tropics towards the poles, descending in 

 the temperate zone as west and south-west winds. These 

 are now strongest in the southern hemisphere, and, passing 

 everywhere over a wide ocean, they supply the moisture 

 necessary to produce the enormous quantity of snow which 

 falls in the Antarctic area. During the period we are now 

 discussing, however, this state of things would have been 



