GROUND WATER 94. General relations. Water which falls upon the earth as rain is disposed of in three ways, a part of it is evaporated, a part remains as surface water in lakes and rivers, and a third part enters the ground to remain for a longer or shorter period of time. The amount of water in the ground depends, therefore, directly upon the amount of rainfall, and less directly upon the character of the topography and the texture of the soil. The more gently rain falls, the more of it enters the ground, and sometimes all of it is absorbed. If the surface of the country is flat, the ground will absorb more rain than if it is hilly, and if the soil is loose and open, it will take up more water than if it is compact. Taking the world as a whole, 22 per cent of the precipitation enters the rivers, and the remaining 78 per cent is divided between that which is evaporated and that which enters the ground. Certain rivers in New South Wales take only 2 per cent of the precipitation, while some of the New England streams carry over 50 per cent. Ground water does not occupy reservoirs, but occurs in minute quantities between the grains of rock and in joints, crevices, and planes of stratification. Chalk may hold two gallons of water for every cubic foot; sandstones may hold 20 to 30 per cent of their weight; and even dense granite contains an appreciable amount. The aggregate supply is therefore very large. In the sandstone underlying parts of Minnesota and Wisconsin there is water enough to make a lake covering these states to a depth of 50 to 100 feet. In the joints of the crystalline rocks of Connecticut there is I cubic inch of water for 125 cubic feet of rock. If all the ground water were squeezed from the rocks, there would be enough to cover the entire earth with an ocean of fresh water over 100 feet in depth. This great amount of water is near the surface, its amount and accessibility varying with all the factors which determine climate. The level below which soil, subsoil, and rock are always saturated is the water table-not a horizontal surface, but a plane which adjusts itself to the topography. It is at less depth in valleys than on hillsides, and it bends upwards and downwards with hills and valleys, and advances toward or recedes from the surface in harmony with variations in precipitation. Streams whose beds reach below the water table are perennial, and wells must be sunk below it to yield water. In humid regions the water table is from 10 to 40 feet below the surface, but in arid regions it may be only after years of irrigation that the table comes within reach of field crops. Water contained in the ground does not remain stationary, but has a drift or underground flow, usually in a definite direction. This flow is rarely in the form of distinct streams; and only in limestone regions where underground drainage has been established do such phenomena occur, as in parts of Kentucky, where the underground flow is so large that surface channels are dry over many square miles. Generally the water finds its way between the grains composing the rock or along partly closed joints, and its movement is excessively slow. With a slope of 10 feet to a mile, ground water will move in fine sand about 52 feet per year, in gravel a mile a year. Petrifaction, mineral springs, and land slides are also phases of ground-water work, and the ore deposits of the world owe their existence chiefly to the same cause. 95. Recovery of ground water. Underground water is returned to the surface naturally by springs and artificially by wells of various sorts. Springs may be outlets for subterranean streams, as Silver Springs, Florida, and Cascade Springs, South Dakota; or they may be located along some crack in the earth's crust and thus be outlets for deeply buried waters, like the hot springs of Virginia, Arkansas, and Nevada; or, finally, they may occur at the contact between strata of open-textured and water-tight rocks. In springs of this type, including probably 90 per cent of their total number, the surface water penetrates the upper beds down to some impervious layer, then follows this layer out to the surface. Wells are holes which reach into the ground below the water table and, accordingly, vary in depth with the character of the soil, the rainfall, and other factors. Most wells are shallow, for the water table in humid regions is near the surface, and porous spaces and open joints decrease in number with the depth. Ordinary wells receive their water supply from the ground immediately adjoining, but in artesian wells the conditions are different. For these it is necessary to have a water-bearing layer, preferably sand or sandstone, over which lie strata practically impervious, such as shale or dense rocks of other types. Water must have access to the pervious layer, but not necessarily near the location of the well. If the waterbearing stratum is exposed at a point higher than the well, the water will be under pressure and will rise through the upper confining layer. Thus in South Dakota a saturated sandstone bed exposed in the Black Hills is covered by clays and shales as it extends eastward. When the clay is pierced at elevations lower than the Hills, the water rises freely and supplies wells even at a distance of 350 miles from its source. The coastal plain of Texas is another excellent artesian area, as is also the New Jersey plain. In the latter case a water-bearing bed is so completely inclosed that wells sunk on barrier beaches, as at Atlantic City, pass through salt water and reach fresh water which has traveled through rock from the western border of the state. Artesian wells do not manufacture water, and since the supply in the ground may be exhausted, care should be taken to husband the supply, particularly in arid regions. 96. Ground water and health. Ground waters carry pollution in the same manner as surface streams, and there is no basis, in fact, for the notion that "running water purifies itself within a mile," or that water from deep sources is pure." The typhoid epidemic at Lausanne, Switzerland, in 1872, was caused by germs which passed through more than a mile of glacial débris, and at Montclair, New Jersey, disease germs carried by ground water were pumped from a well 400 feet in depth. Location with reference to surface conditions and direction of underground flow are more important than depth. Owing to sewage contamination, a commission appointed by the New Haven Chamber of Commerce recommended that "all wells in the city be abandoned for domestic supply." Typhoid epidemics are usually caused by polluted waters, and it is recognized that "the prevalence of typhoid in cities is a true index of the quality of the water supply." 97. Location of settlements. The ground-water supply has had much to do with the growth of cities and the location of farms and ranches, as well as of oases. Some districts to the east of Paris, in the Marne valley, for example, those near Verdelot, show villages and farmhouses widely scattered because the ground water is kept near the surface by the presence of an impermeable layer. Farther south, in the Seine valley, at Origny-le-Sec, the water sinks through permeable rock, requiring deep wells, and the villages are accordingly grouped where such wells have been dug. The rocks underlying the suburban districts north of London do not retain the rainfall, and the growth of the city in that direction was retarded until water was conveyed artificially. On the western plains and in the deserts of New Mexico, Arizona, and southern California many villages and farms owe their existence to the presence of a spring or well, and one of the striking features of an arid region is the presence of artificial oases, -spots of green surrounding an artesian well in the midst of a monotonous stretch of desert land. CHAPTER V ATMOSPHERE AND CLIMATE ATMOSPHERE 98. General and physical. The atmosphere is the entire gaseous envelope which surrounds the earth. It retains and modifies the heat of the sun, distributes moisture, and makes it possible for the forms of life with which we are acquainted to exist on the planet. Without an atmosphere the earth would resemble the moon, a cold, barren lifeless sphere. The air extends to a height of several hundred miles, but one half of it is within 3 miles of the earth, and 99 per cent within 25 miles. The inhabitants of Tibet, on a plateau 15,000 feet in elevation, have scarcely more than half the amount of air used by dwellers at the seashore; and climbers in high mountains above 20,000 feet find it almost impossible to live on the small amount of air present.1 Although invisible, air has a certain definite weight, varying with the altitude. At sea level the weight of the atmosphere (i.e. its pressure) is 14.7 pounds for every square foot of surface, and, accordingly, the pressure on a man's body amounts to several tons. The density of air is proportional to the pressure to which it is subjected; it is also elastic and readjusts itself with great readiness. These properties make air valuable as motive power for machinery, and account, among other things, for its extensive use in air brakes. The temperature of the air is controlled by the sun, and air of different degrees of warmth is distributed over the earth in accordance with the effectiveness of the sun's rays. The atmosphere is, however, only slightly warmed by the direct heat of the sun, a much greater effect being produced by the rays which are absorbed and radiated by the earth. The air thus receives the sun's heat both as it comes to the earth and as it leaves the earth, and temporarily retains about 70 per cent of the whole amount derived from the sun. The upper air is everywhere cold, and in the space beyond the atmosphere the temperature remains constantly below zero. 1 St. Elias, 18,024 feet, and Illimani, 22,500 feet, have been ascended, and in 1908 the Duke of the Abruzzi climbed 24,600 feet in the Himalayas. The atmosphere consists almost entirely of five gases, in the following proportions: nitrogen, 76.95; oxygen, 20.61; water vapor (average), 1.40; argon, I; and carbon dioxide (average), 0.03. Of these nitrogen is the element which is responsible for most of the pressure and density of the air; yet, although it is present in such large quantities, living forms have learned to make little use of it. It does, however, make possible the flight of birds, and in a roundabout way furnishes food to plants (§ 73). The oxygen of the air supports life in plants and animals, and is, perhaps, the most effective agent in the formation of soil. For some reason, animals have learned to use diluted oxygen rather than the more abundant nitrogen, and so dependent are they upon it, that without an adequate supply death ensues. Carbon dioxide, although present in small amounts, plays a most important part in nature (§ 71). A dry atmosphere consists almost entirely of the two gases, nitrogen and oxygen, but water vapor is nearly always present in amounts which depend upon temperature. For instance, at a temperature of 20° F. a cubic foot of air will hold 1.235 grains of water vapor, at 40° F. it will hold 2.849 grains, and at 100° F. it will hold 19.766 grains. Humidity is the general term used to express the percentage of water vapor in air; the amount actually present is the absolute humidity, and the proportion that this amount bears to the quantity air can hold at a given temperature is the relative humidity. Relative humidity is the important factor, so far as man is concerned, for it determines whether the air is dry or damp, "close," or "muggy." A dry climate is one in which the atmosphere is far from saturation, i.e. could hold much more water vapor; but in a damp climate there is always an approach to saturation, although the actual amount of water vapor present may be small. Damp air in the arctics, for instance, may contain but one tenth of the water vapor held by damp air in the tropics. The water vapor of the atmosphere may be condensed by cooling and take the forms recognized as fog, clouds, rain, snow, hail, dew, or frost. Condensation near the surface produces fog, at higher altitudes clouds, and both of these forms may disappear when the air grows warmer, thus gaining an increased capacity for holding moisture. When the condensation of water vapor produces particles too large to be supported in the air, these fall as rain or snow, the form depending upon whether the condensation takes place above or below the freezing point. Sleet is partly frozen snow, and hail is made up of raindrops which have been carried so far upward that they are frozen before they fall to the earth. Dew and frost form on the |