TJie Winds. 24>7 



Supposing, now, the boiler of the cryophorus to "be at the 

 equator, and its condenser at the poles, a distribution of rain 

 in a horizontal direction from the equator would take place. 

 But the winds alternately carry moisture outwards from the 

 equator, and act as condensers returning from the poles ; and 

 the process of lateral condensation is also modified by ele- 

 vated mountain ridges and irregularities of the land. Upon 

 the south-west coasts of Europe, autumn rains are abundant, 

 occurring with the season of the south-west wind. The effect 

 of mountain ranges in arresting moisture, is illustrated by 

 the enormous quantities of rain which fall at Coimbra, Borrow- 

 dale, and Bergen, on the coast : whilst Madrid, surrounded by 

 high mountains on the table-land of Castile, has a rainfall of 

 only ten inches in the year. At Lyndon, in the centre of 

 England, the rainfall is below the average. Only flower seeds 

 can be cultivated in some parts of the British Islands, on 

 account of the deficiency of moisture ; but at Seathwaite in 

 Cumberland, at Toronsay in the Isle of Mull, and at Portree 

 in the Isle of Shye, the fall of rain is more than 100 inches 

 annually. Other examples, to illustrate the effect of mountain 

 ranges in precipitating the moisture of the wind, will be bor- 

 rowed from tropical climates, in describing the winds of those 

 climates, called monsoons. 



In the highest regions of the atmosphere, where condensa- 

 tion constantly takes place, moisture is deposited in the 

 solid form, and accumulates from year to year, as snow, on 

 mountain tops, even at the hottest parts of the equator. 

 Large masses detaching themselves from its sides, rush down 

 the steeps in avalanches ; but water melted from the surface 

 by the sun, converts a large portion of the interior into ice. 

 Glaciers are thus formed, whose ponderous masses slowly 

 descend the ravines, until they melt away below in torrents 

 that water innumerable valleys. Above the height where the 

 mean temperature of summer is the same as the freezing-point 

 of water, there is a perpetual surface of fresh fallen snow. 

 Below this point, the glaciers and mountain slopes, in 

 summer, are free from snow. The level in question is called 

 the " snow-line/' or level of perpetual snow. 



The following table of the measured height of the snow- 

 line in widely different latitudes, from a list of places contained 

 in Humboldt's work on Central Asia, enables us to calculate 

 the decrement of temperature, in ascending, from the mean 

 heat of summer on the plains, to a temperature of 32° (F.) at 

 the height of perpetual snow. The corresponding decrements 

 for equal elevations, determined by Mr. Glaisher in his recent 

 balloon experiments, when the sky was clear, are next entered 

 in the table for comparison. 



