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Page:Popular Science Monthly Volume 54.djvu/460

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THE 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 degree of heat on which the enormous sickness and death rate of the people depend? Discussing the subject broadly from this standpoint, 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 favorable for the habitation of man.

It requires but little knowledge of the physical forces which 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 become 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 unknown.

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 winter 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 difference 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 difference of twenty, thirty, and forty degrees, and in the soil a difference 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.