B. TERRESTRIAL PHYSICS AND METEOROLOGY. 75 



climate. In his article he remarks that forests have a chem- 

 ical, physical, physiological, and mechanical action on the 

 climate of a country. In regard to the physical action, 

 while the foliage of woods allows much less rain-water to 



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reach the ground than in unwooded land, this is more than 

 compensated by the difference of evaporation in the two 

 cases, that of the open fields being nearly five times as great 

 as that of the woods. The melting of snow, too, is retarded 

 by forests, thus causing a more gradual outflow of the water. 

 Again, forests are obstacles to atmospheric movements. An 

 air current meeting a wood is compressed and forced up- 

 ward, so that it yields part of its moisture in the form of 

 rain. Forests also protect crops against the winds; and it 

 is an established fact that thunder-storms are less frequent 

 and violent in w^ooded regions than in open countries, as the 

 trees draw from the atmosphere the electricity it contains, 

 which accumulates on regions that are bare. Forests, too, 



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have a decisive action as regards the formation of hail, hail- 

 storms occurring but rarely in a wooded region. A case has 

 lately been noted where a violent hailstorm on approaching 

 and crossing a forest ceased to produce hail, but resumed its 

 formation on passing to the unwooded country beyond. 18 

 A, June 11,324. 



NOTE OX THE VEETICAL DISTRIBUTION OF TEMPEEATUEE ON 



THE OCEAN. 



Mr. Buchanan, chemist on board of the Challenger, writes 

 that the effect of the changing seasons on the temperature 

 of the sea-water seems to him not to have received sufficient 

 attention. During the whole period of the heating of the 

 water it has, from its increasing temperature, been steadily 

 becoming lighter, so that the communication of heat to the 

 water below by convection has been entirely suspended. It 

 has, also, by evaporation, become denser than it was before 

 at the same temperature. During the approach of winter, 

 the superficial water having cooled, sinks through the warmer 

 water below it, until it reaches the stratum having: the same 



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temperature as itself. Nor does it stop there, but continues 

 to sink, owing to its density, carrying its temperature with 

 it to the lower colder layers. The result is that we have 

 during the winter a heating effect going on in the lower re- 



