442 POPULAR SCIENCE MONTHLY. 



cessive heat can not occur. The real problem to be solved may be 

 thus stated : How can the temperature of the city of New York be so 

 modified during the summer months as to prevent that extreme de- 

 gree of heat on which the enormous sickness and death rate of the 

 people depend? Discussing the subject broadly from this stand- 

 point, it becomes at once evident that w T e must employ those agencies 

 which in the wide field of Nature are designed to mitigate heat and 

 purify the air and thus create permanent climatic conditions favor- 

 able for the habitation of man. 



It requires but little knowledge of the physical forces w T hich 

 modify the climate of large areas of the earth's surface to recognize 

 the fact that vegetation plays a most important part. And of the 

 different forms of vegetation, trees, as compared with shrubs, plants, 

 vines, and grasses, are undoubtedly the most efficient. This is due to 

 the vast area of surface which their leaves present to the air on a 

 very limited ground space. The sanitary value of trees has hitherto 

 been practically unrecognized by man. With the most ruthless hand 

 he has everywhere and at all times sacrificed this most important 

 factor in the conservation of a healthful and temperate climate. He 

 has found, too late, however, that by this waste of the forests he 

 has by no means improved his own condition. The winters have be- 

 come colder, the summers hotter; the living springs have ceased to 

 flow perpetually; the fertilizing streams have disappeared; the earth 

 is deeply frozen in winter and parched in summer; and, finally, new 

 and grave diseases have appeared where formerly they were un- 

 known. 



It is well understood that the temperature in a forest, a grove, 

 or even a clump of trees, is cooler in summer and warmer in win- 

 ter than the surrounding country. Man and animals alike seek 

 the shade of groves and trees during the heat of the day, and are 

 greatly refreshed and revived by the cool atmosphere. The differ- 

 ence between the temperature of the air under and among the 

 branches of a single tree, densely leaved, and the surrounding air, 

 on a hot day, is instantly realized by the laborer or traveler who 

 seeks the shade. The thermometer in the sun and shade shows a dif- 

 ference of twenty, thirty, and forty degrees, and in the soil a differ- 

 ence of ten to eleven degrees. The reverse is true in winter. The 

 laborer and traveler exposed to the cold of the open country find 

 in the forest a degree of warmth quite as great as in a building but 

 imperfectly inclosed. Railroad engineers inform us that they have 

 occasion to use far less fuel in passing through forests in winter than 

 in traversing the same distance in the open country. When the 

 ground in the fields is frozen two or three feet deep, its temperature 

 in the forest is found above the freezing point. 



