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TREES FROM A SANITARY ASPECT.

BY CHARLES ROBERTS, F.R.C.S., ETC.

As this is the usual time of the year for planting, pruning, and removing forest trees and shrubs, it is a fit time for considering the influence which trees exert on the sanitary surroundings of dwelling places.  The recent parliamentary report on forestry shows that trees are now of little commercial value in this country.  And we may conclude, therefore, that they are chiefly grown for picturesque effect, and for the shelter from the sun and winds which they afford.

The relation of forests to rainfall has been studied by meteorologists, but little attention has been given by medical climatologists to the share which trees take in determining local variations of climate and the sanitary condition of dwellings, notwithstanding they play as important a part as differences of soil, of which so much is said and written nowadays.  This remark does not apply to large towns, where trees grow with difficulty and are comparatively few in number, and where they afford a grateful relief to the eye, shade from the sun, and to a very slight extent temper the too dry atmosphere, but to suburban and country districts, where it is the custom to bury houses in masses of foliage—­a condition of things which is deemed the chief attraction, and often a necessary accompaniment, of country life.

Trees of all kinds exercise a cooling and moistening influence on the atmosphere and soil in which they grow.  The extent of these conditions depends on the number of trees and whether they stand alone, in belts, or in forests; on their size, whether tall trees with branchless stems or thickets of underwood:  on their species, whether deciduous or evergreen; and on the season of the year.  The cooling of the air and soil is due to the evaporation of water by the leaves, which is chiefly drawn from the subsoil—­not the surface—­by the roots, and to the exclusion of the sun’s rays from the ground, trees themselves being little susceptible of receiving and radiating heat.  The moisture of the atmosphere and ground about trees is due to the collection by the leaves and branches of a considerable portion of the rainfall, the condensation of aqueous vapor by the leaves, and the obstruction offered by the foliage to evaporation from the ground beneath the trees.

The experiments of M. Fautrat show that the leafage of leaf bearing trees intercepts one-third, and that of pine trees the half, of the rainfall, which is afterward returned to the atmosphere by evaporation.  On the other hand, these same leaves and branches restrain the evaporation of the water which reaches the ground, and that evaporation is nearly four times less under a mass of foliage in a forest, and two and one-third times under a mass of pines, than in the open.  Moreover, trees prevent the circulation