178 The Realm of Nature CHAP. 



240. Circulation of Water by Wind. Apart from 

 producing waves, the wind slips the top layer of water 

 before it as one might slip a card from the top of a 

 pack, and although it can act only on a very thin film 

 a new surface is constantly exposed, and a steady breeze 

 causes a great surface drift. Mr. J. Aitken appears to have 

 been the first to point out the importance of this action in 

 disturbing the deeper layers of water. Dr. Murray, by a 



series of temperature ob- 

 ,A servations on Loch Ness, 

 showed how rapidly wind 

 acting in this manner on 

 the surface of a deep lake 

 could completely alter the 

 distribution of the water. 5 



1*IG. 35. Circulation of water by wind. . . 



The light lines and figures show dis- The explanation of his 



tribution of temperature before, and n K Q r vptinnc: cppmc rr \\e- 



the dark lines and figures distribution Observations S 



of temperature after, the wind has been as follows I On a Calm 



blowing in the direction of the long summer day ^ ]ake QQn _ 



tains a surface layer about 



1 5 fathoms deep, the temperature of which is from 60 to 

 50, floating upon 100 fathoms of water, the temperature 

 of which is from 50 to 40. When strong wind blows 

 steadily along the length of the lake from A to B the 

 surface water is driven toward B, where the wind heaps it 

 up, but the greater pressure of the heaped-up water causes 

 the lower layers at B to move off toward A, and thus the 

 whole end of the lake-basin at B is filled with the warm 

 water that had been resting on the surface, while the cold 

 water formerly filling the depths rises against the shore 

 at A, as represented by the arrows. If the wind lasts long 

 enough the water will be thoroughly mixed and the tempera- 

 ture made uniform throughout ( 228). 



241. Effect of On-shore and Off-shore Winds. 

 Bathers know that in summer the sea is colder when the 

 wind is blowing from the land than when it is blowing from 

 the sea. The reason is that the wind blowing from the sea 

 (an " on-shore " wind) drives the surface water, which has 

 been heated over a wide area, in toward the shore, on which 



