=Opening the Outflow=, in order that water may not remain stagnant in the soil, is occasionally required. This may be done by clearing water-courses, removing obstructions, and forming fresh channels. The provision of sufficient =surface-drains= to carry off ordinary water and storm-water helps in drying the soil of urban districts. =Vegetation= tends to diminish dampness of soil by causing rapid evaporation, and at the same time uses up the organic matter in the soil. Certain plants are more active in producing these effects than others: the _Eucalyptus_ genus, including many species, and represented by the well-known _blue-gum tree_ of Australia, is noted for its power in this respect; and the common sun-flower, which is very easy of cultivation, has a powerful influence in the same direction. CHAPTER XXXIV. CLIMATE AND WEATHER. =The Climate= of a country has an important influence on the health and character of its inhabitants. The character of a climate depends on four main conditions:— 1. The distance from the equator. 2. The height above the sea. 3. The distance from the sea. 4. The prevailing winds. There are other conditions which are of subsidiary importance, but which have great influence in modifying the climate of any given locality. Thus:— 5. The nature of a surface—its aspect, shelter, slope; the colour of the soil or rock, the reflection from rocks or sheets of water, and the influence of vegetation. 6. The cultivation of the soil. 7. The drainage of marshes and damp soils. 8. The planting and clearing away of forests. =The Distance from the Equator= is the most important factor in relation to climate. The sun’s rays become less powerful as they fall more obliquely, in travelling from the equator. This primary factor in producing climate is largely modified, however, by the relative distribution of land and water, and by the character of the prevailing winds of a district. =The Elevation= of a locality affects the temperature and the barometric pressure, both falling as the height is increased. The amount of fall varies with the latitude of the place, with its situation in regard to surrounding districts, the degree of moisture of the air, the presence of winds, the hour of day, and the season of the year. It is usual to allow 1° Fahr. for every 300 feet of ascent above the level of the sea, and one-thousandth part of an inch depression of the barometer for every increase of one foot in height.