Page:Popular Science Monthly Volume 2.djvu/210

 minimum of the forest being less than that of the field; in other words, the diurnal temperature is more uniform.

But this is a matter involving such complicated and varying conditions, that absolute propositions are open to question, however true they may be with proper qualifications. Rivoli has very recently done much to make our knowledge of this subject definite. His observations were carefully made, under circumstances which eliminated, as much as possible, all disturbing conditions; there being no body of water near, the country level, and the wind having a fair sweep in all directions. We will state the results of his investigations as briefly as possible.

Influence of Forests on Winter Temperature.—In the winter-time, the simplest relations of the forest to temperature prevail. During this season of the year, when the wind passes into the north and becomes colder, the forest warms it; when it passes into the south and becomes warmer, the forest cools it. During winter the forest plays the role of a bad conductor, and acts as an equalizer of temperature.

Influence of Forests on Summer Temperature.—In summer the case is not so simple, owing to evaporation from the surface, transpiration through the leaves, and radiation from them. At this season of the year the atmosphere in the forest is usually warmer during the nights and colder during the day than in the open field. The night is warmer in consequence of the obstruction which the mass of foliage presents to radiation from the surface beneath it; the day is cooler in consequence of the transpiration of vapor from the leaves, and the obstruction interposed between the surface and the direct rays of the sun. In the summer, as well as in the winter, the forest usually acts as an equalizer of the temperature of the atmosphere.

While, however, this is usually the case, there are exceptions. During nights when it is calm, radiation from the leaves of the trees may cool particles of air, which, descending toward the surface, form just before daybreak a stratum of the atmosphere below, which is colder than if the region had been destitute of trees. It is within the experience of most cultivators of the soil that frosts sometimes strike hardest near a wood.

To the fact that under certain circumstances a forest may cause frost in the fields near by, we may add the qualification that this can only occur in the case of white frosts, and that whenever there is motion of the atmosphere, and the wind a cold one, the influence of the forest is always protective. An orchard sheltered by a wood may escape unhurt, while another in the same neighborhood not so protected may suffer the loss of its entire crop. This is believed to be not a very uncommon occurrence in the case of peaches.

If the fruit-growing interest of the country were to state its account with the forest, we should suppose it to be something like the following: