THE effects of a forest-covering on the general physical conditions of a country are very great. Before describing our North American woods it will be desirable for us to note some of the most conspicuous ways in which this influence is exerted. The most immediately recognizable action of forests is that which they have upon the temperature conditions of a country. All who are familiar with the woods have noted the singular coolness of their shade in the heated season, and the relative warmth in the winter time which is found beneath their branches even when they are bare of leaves. In summer there is often a difference at midday of as much as 8° or 10° Fahr. between the temperature within a forest and that of the neighboring open fields. In winter the variation is less, yet it is still considerable. A careful study has shown that the equalizing effect of forests composed of different species varies greatly. It is least in the case of the narrow-leaved species, such as the pines and firs, and is greatest in woods where the shade is due to the broad-leaved forms, which have a dense foliage such as the beeches afford.
The conservative effect of the forest on temperature in regions of considerable snowfall is also great, for the reason that the snow within their recesses is preserved from melting in the warmer days of the winter, and may retain a considerable depth long after it has disappeared on the open ground. Thus forests tend to make the climate of a country, as far as its temperature is concerned, much more uniform than it would otherwise be. Another important influence arising from the presence of great forests is found in the restraint which they put upon the movements of the rain water. When the rain descends upon the trees a good deal of the water is retained on the leaves, branches, and trunks, and does not find its way to the ground, but evaporates from the mass of the vegetation. It is likely that in the growing season, especially after a period of drought, near an inch of rainfall may,