1 See any standard Physical Geography.
Clouds, whether brought by these currents of air or formed nearer at hand, modify the climate by screening the earth from the direct rays of the sun, and by preventing rapid radia tion of the heat when the direct rays are with drawn.
Ocean currents affect the climate of the regions near which they flow, causing, for example, the difference between the tempera ture of France and that of Labrador in equal latitudes, and giving to the British Islands and Norway, to Japan and Alaska, a much warmer and moister climate than is ordinarily to be found in lands so far from the equator. The oceanic currents arise from the unequal heating of water, as those of the atmosphere arise from the unequal heating of air. Among the minor but still important regulators of climate is the presence of forests and other vegetation. A very great quantity of water is given to the air by the leaves of trees and grasses, and the moisture of the climate is thus while the temperature is at 1 Gaye, The Great World's Farm, Chapter IX.
the same time lowered by the using up of the heat in the process of evaporation.
The agencies that unite to produce climate are interwoven in such various ways that lines connecting places of equal temperature 1 are by no means parallel with the earth's equator. The extremes of temperature are crowded together much more closely in the Old World than in the New. In some parts of the Eastern Hemisphere differences are met with in a day's journey which are as great as those that the traveller would experience in going from Alaska to Mexico. In North America the general in crease in elevation toward the south partly off sets the effect of the more vertical rays of the sun in the equatorial zone, thus accounting for the slower increase in temperature. When we say of a particular place in the temperate zone that it has a fine climate, we mean that it has a fair proportion of days in which there is bright sunshine ; that its summer temperature is neither so high as to kill vegetation and to render the life of man insupportable, nor so low as to pre These lines are called isotherms. They are shown upon the maps issued by the weather bureau.
vent the ripening of grains and fruit ; that there is a liberal supply of moisture, sufficiently dis tributed throughout the year to avoid the dan gers of drought and flood ; that violent storms do not occur, or are infrequent ; that there is sufficient moisture in the atmosphere in the form of clouds to serve as a screen from undue heat and cold, but not so much humidity as to increase the discomfort of summer ; and, finally, that the atmosphere is not contaminated by foul vapors and disease germs.
Such a climate will be conducive to health and vigor, and if other conditions are equally favorable, it will give ample rewards for labor spent upon the cultivation of vegetable life. Its natural vegetation may be less luxuriant, but the conditions of economic life are on the whole more favorable than in the hotter and moister climate of a tropical zone. It is not at all es sential that all of the conditions enumerated should be present in order to constitute a desir able economic environment. Man may succeed in his struggle for a living even where the cli mate is not equable, where the summers are in tensely hot and the winters bitterly cold, where the winds bring little moisture and there are no rains for months in succession. This is only to say that climate is but one element in the ma terial environment, and that we must consider also its other features.
Among these none is of greater significance than the soil. In the present chapter it is the origin and composition of soils with which we are concerned. In a future chapter, on the making of goods, it will be pointed out that soils can be modified in character, and their use ful qualities renewed as they are worn out. Soil consists mainly of clay, sand, and vegetable mould ; or of a mixture from which one or more of these elements may be absent. A large number of other ingredients, such as iron, lime, and potash, although present in comparatively small quantities, are important because they are the very elements 1 that enter into the composi tion of plants. Clay is formed by the decompo sition of feldspar, a mineral which also contains the chemical ingredients of potash and other valuable plant foods. Sand is the result of the decomposition of rock comparatively poor in 1 The word is not here used in a chemical sense.