XII. 



ICE AND ITS WORK. 



By JoHx MoRisox, M.D., F.G.S. 



Read at Watford, IGth December, 1892. 



Abrido^ed. 



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Water, as wc all know, is by far the most potent geological 

 agent in mollifying the snrface of the earth, and the work done by 

 ice, which is solid water, is scarcely less important than that which 

 is accomplished by water in the liquid state. I will here treat of 

 the work of ice, and the traces or tool-marks which it has left 

 behind it in many parts of the world, especially in our own country 

 and our own county. 



The sun's heat is continually drawing up vapour in vast quantities 

 from all the expanses of water on the earth's surface ; and the water- 

 surface of our globe is equal to about fths of its entire area. The 

 hotter the weather is, the more watery vapour is drawn into the 

 air, but there is always a certain amount of evaporation, however 

 cold the weather may be. Our atmosphere thus always contains a 

 greater or smaller amount of vapour of water. When the air is cooled 

 down below a certain point, called the dew-point, this vapour con- 

 denses on the minute particles of dust of which the air is full, and 

 forms mist or cloud. These fine watery particles either remain 

 suspended in the air, or, uniting together, fall to the surface as 

 rain, hail, or snow. In those parts of the world where the mean 

 annual temperature is below the freezing-point, the condensed 

 moisture falls chiefly as snow. This is the case in the Polar 

 Regions, and also on mountain summits which are elevated suffi- 

 ciently into the upper and colder regions of the atmosphere. In 

 these places the snow covers the ground and remains more or less 

 unmelted throughout the year. A line called the snow-line, vary- 

 ing in altitude in different parts of the earth's surface, may be 

 drawn, above which the snow remains unmelted in summer. This 

 line is close to the sea-level in the polar regions, and rises gradually 

 as we approach the equator to a height of 18,000 or 19,000 feet. 

 In our country it would be about 5,000 feet. When a current of 

 warm air laden with moisture passes over cold mountain tops, it 

 becomes chilled, and the vapour is condensed as I have described, 

 and may fall down as rain or snow. There is much greater pre- 

 cipitation of moisture on mountains than on plains on account of 

 their greater coldness, and on the loftier summits above the snow- 

 line this precipitation takes the fonn of snow which remains in 

 great part unmelted throughout the year. 



Two things are necessary to produce an accumulation of per- 

 petual snow — (1) acertain degree of elevation above the sea-level- 

 varying in different latitudes, and (2) the presence of currents of 

 air more or less warm and laden with moisture. The snow which 

 falls on cold mountain tops cannot flow down as rain would. 



