GENERAL IDEAS. 7 



that of mountains. It varies with the elevation of 

 the plain, the nature of the soil, the proximity or 

 remoteness of the sea and of mountain chains, as 

 well as with the direction of the latter ; it is further 

 modified by the presence or absence of large sheets 

 of surface water, and of forests. 



, Surface water by its evaporation lowers the 

 temperature in summer. As it cools down slower 

 than the atmosphere, it may diminish the cold at the 

 beginning of winter ; but once in a solid state, its 

 more active radiation adds to the severity of the 

 cold. Finally, in the spring it absorbs heat slowly, 

 and thus too it lowers the temperature. Watery 

 vapour in the shape of fogs intercepts the solar rays, 

 and is apt to bring on frost. The presence of surface 

 water renders the atmosphere moist, and lastly it 

 opposes no obstacle to the wind, which thus retains 

 all its force. 



Although forests prevent the evaporation of water 

 by the action of their cover, still their foliage diffuses 

 much watery vapour into the atmosphere, and thus 

 the temperature is lowered during the season of 

 vegetation. In winter they impede terrestrial 

 radiation and in consequence diminish the intensity 

 of the cold. But a series of experiments not yet 

 completed, proves that the mean annual temperature 

 is rather less inside the forest than it is outside, and 

 tends to show that the effect of forests as regards 

 heat, is chiefly to prevent and to lessen sudden 

 changes of temperature. The atmosphere contains 

 more humidity in wooded countries than in places 

 which are denuded of forest vegetation ; it rains 



