360 HUMAN BIOLOGY 



and the air. The amount of this current is dependent upon 

 the ionization of the atmosphere, which in turn is affected 

 by the presence of radio-active substances, by ultraviolet 

 radiation and probably by the penetrating radiation recently 

 studied by Millikan. The relation of these electrical prop- 

 erties of the air to health and comfort offers an interesting 

 held of study. At present our knowledge concerning this is 

 imperfect. 



As regards urban and rural atmospheres, great differences 

 are at once apparent. The physical properties, temperature, 

 humidity and movement, are determined, of course, by 

 latitude, altitude and relation to seashore and mountains, 

 but under otherwise equivalent conditions, the "open air" 

 of the country has advantages, especially in the summer 

 time. 



A characteristic of large cities is the mass of heated brick, 

 stone and concrete that reflects much of the sun's heat 

 back upon the dwellers and retains what heat is absorbed, 

 giving it out in the night hours, preventing the natural 

 coohng that comes in the country with the setting of the 

 sun. Air movement is lessened by tall buildings, and the 

 human output of humidity and heat becomes a distressing 

 factor in crowded places. 



Whereas in the city the radiant heat of the summer sun, 

 striking the masonry of buildings and paved streets, is either 

 reflected, adding to the immediate discomfort, or absorbed 

 and stored, to be returned during the night-time, in the 

 country it is to a large extent absorbed and neutrahzed 

 by green fohage. The full significance of this phenomenon 

 can best be shown by a brief mathematical computation. 



It has been estimated (Bailey) that an acre of beech 

 trees, 400 to 600 trees, will evaporate about 2,000,000 

 pounds of water during the season, or let us say 10,000 

 pounds per day. The heat absorbed by this amount of 

 evaporation amounts to 6^ small calories per square centi- 

 meter per day. 



The solar constant, as defined by the Smithsonian Insti- 

 tution, represents the quantity of heat that would be received 

 from the sun if there were no atmosphere. On a clear day, 

 at sea level, the actual heat received is of the order of 90 



