If the slopes of a hill are covered with trees the temperature of its sides and base are considerably increased, as the trees obstruct the descending currents of cold air. The frosts of winter are felt most severely in localities where the slopes above them are destitute of vegetation, and especially of trees. It follows that in any given locality, the best protection against the winter cold is ensured by a dwelling situated on a slope a little above the plain or valley from which it rises, with a southern exposure, and sheltered by trees planted above it. Such local conditions should always be carefully enquired into, when a choice of site is possible, as the temperature of one part of a neighbourhood may differ by several degrees from that of another part near at hand. This is particularly important in the case of invalids. =Forests= tend to modify a climate, and mitigate its extremes, whether situated on the slopes of mountains or on plains. In America, as elsewhere, the effect of destruction of forests has been to produce greater variation in the annual rainfall, to lengthen periods of drought, and to increase the power of floods and cloud bursts. Trees are heated and cooled by radiation like other bodies, but from their slow conducting power, the periods of their maximum and minimum temperatures are not reached for some hours after the same phases of the temperature of the air, and the effects of radiation are not confined to a small surface on the soil, but distributed to the level of the tree-tops. For these reasons, trees make night warmer and day cooler, thus giving to forest districts something of the character of an island climate. Evaporation occurs slowly from the damp soil beneath trees, as it is screened from the sun, and the trees prevent a free circulation of wind. Hence the relative humidity and rainfall are increased. At the same time forests mitigate the disintegrating effect of the rainfall on the soil. =Ground covered with Vegetation= has a more uniform temperature than bare soil, the effect being much the same as that of forests, though on a smaller scale. All growing vegetation evaporates a large quantity of water. A plant evaporates 200 pounds of water while it forms one pound of woody fibre; the effect of a forest must, therefore, be enormous. At the same time, vegetation, and especially trees, retain moisture in the soil. The water-supply of barren regions may be greatly increased by planting trees. _The absence of vegetation_ leads to extreme fluctuations of temperature. An extent of sand, for instance, raises the temperature of the air greatly during the day, as it is a bad conductor; but at night, radiation is very great, and the temperature falls accordingly.