THE SUMMER HEAT OF CITIES. 443 



Forests and even single trees have, therefore, a marked influence 

 upon the surrounding atmosphere, especially during the summer, 

 and they evidently tend to equalize temperature, preventing extremes 

 both in summer and winter. Hence they become of immense value 

 as sanitary agencies in preserving equality of climatic conditions. 



It is believed by some vegetable physiologists that trees exert 

 this power through their own inherent warmth, which always re- 

 mains at a fixed standard both in summer and winter. " Observation 

 shows," says Meguscher,* " that the wood of a living tree maintains 

 a temperature of from 54° to 56° F., when the temperature stands 

 from 37° to 47° F. above zero, and that the internal warmth does 

 not rise and fall in proportion to that of the atmosphere. So long 

 as the latter is below 67° F., that of the tree is always highest; but, 

 if the temperature of the air rises to 67° F., that of the vegetable 

 growth is the lowest." Since, then, trees maintain at all seasons a 

 constant mean temperature of 54° F., it is easy to see why the air in 

 contact with the forest must be warmer in winter and cooler in sum- 

 mer than in situations where it is deprived of that influence. f 



Again, the shade of trees protects the earth from the direct rays 

 of the sun, and prevents solar irradiation from the earth. This effect 

 is of immense importance in cities where the paved streets become 

 excessively heated, and radiation creates one of the most dangerous 

 sources of heat. Whoever has walked in the streets of ISTew York, 

 on a hot summer's day, protected from the direct rays of a midday 

 sun by his umbrella, has found the reflected heat of the pavement 

 intolerable. If for a moment he passed into the dense shade of a 

 tree, he at once experienced a marked sense of relief. This relief is 

 not due so much to the shade as to the cooling effect of the vaporiza- 

 tion from the leaves of the tree. 



Trees also have a cutaneous transpiration by their leaves. And 

 although they absorb largely the vapor of the surrounding air, and 

 also the water of the soil, they nevertheless exhale constantly large 

 volumes into the air. This vaporization of liquids is a frigorific or 

 cooling process, and when most rapid the frigorific effect reaches its 

 maximum. The amount of fluid exhaled by vegetation has been, at 

 various times, estimated with- more or less accuracy. Hales % states 

 that a sunflower, with a surface of 5.616 square inches, throws off at 

 the rate of twenty to twenty-four ounces avoirdupois every twelve 

 hours; a vine, with twelve square feet of foliage, exhales at the 

 rate of five or six ounces daily. Bishop Watson, in his experiments 



* Man and Nature. G. P. Marsh, New York, 18Y2. 



\ It is interesting to notice, in this connection, the remark of Angus Smith, that a 

 temperature of 54° F. is important in the decomposition of animal and vegetable matter. 

 % Public Parks. By John H. Rauch, M. D., Chicago, 1869. 



