The Movements of the Air are due to the differences in the pressure or density of the atmosphere, the law being that the air always moves from areas of high pressure to areas of low pressure. Atmospheric pressure is determined by temperature and aqueous vapor. Warm air rises, cold air falls. Horizontal currents flow from the cooler regions to replace the volumes which ascend in the wanner. To this cause the trade winds and the well-known land and sea breezes are due. As vapor increases, the density of the air is lessened. Moist air, like warm air, has a tendency to rise. The ascent of moist air lessens the atmospheric pressure, which is shown by a fall in the barometer. When vapor rises to the upper atmospheric regions it expands, cools, condenses, and descends in rain. Unequal and rapid heating of the air, and the accumulation of aqueous vapor, and perhaps somqipflnences not understood, create great disturbances in pressure, resulting in storms, lrurricanesc and cyclones. The fall o? a tenth of an inch in an hour in the barometer is usually followed by violent storms. When atmospheric pressures are widely different in neighboring localities the wind will move from the area of high to that of low pressure, and if the difference in pressure be great, a fierce storm is likely to occur. The average pressure of the air in motion is rated as follows: In a calm, no movement, no pressure; light breeze, 14 in. an hour. 1 lb. to the sq. foot; strong breeze, 42 in. an hour, 9 lbs.; strong gale, 70 m, an hour, 25 lbs.; hurricane, 84 in. an hour, 36 lbs. The changes produced by the air are both chemical and mechanical, and often inseparably united. If chemical, they appear in oxidation (rust) of metals, in the absorbtion of carbonic acid by rocks, and the 'production of earthy carbonates and bicarbonates, which promote the process of decomposition. Dry air has little oxidizing power; moisture is needed for the process. Every housewife knows that iron forks will long• remain free from rust if kept from moisture. In towns the air takes up sul phuric and nitric acid to such an extent as to corrode metal surfaces as well as the mortar of walls, which may often be seen to swell out and drop off, owing to the conver sion of its lime into sulphate.
Expansion and contraction are produced in rocks, as in other substances, by heat and cold. In regions where the range of temperature is great, there is much difficulty in finding building materials that will not be seriously affected by such changes. An engineer of the United States army some time ago investigated the expansion of certain materials. He found that in fine grained granite the rate for every degree of Fahren heit was .000004825; for crystalline marble, .000005668; and in red sandstone, .000009532, or about twice as much as in granite.
.ercezing Water expands and exerts an enormous strain upon any inclosed cavities or walls which contain it. In severe cold, trees often burst from the expansion of frozen sap. The winds, by driving loose sand over rocks, give them a smoother sur face. Prof. Dana asserts that at Cape Cod holes have been drilled in window glass by drifting sand. Cavitiesare sometimes hollowed iu rocks by gyrating sand and fragments of stone. Hurricanes are geological agents, inasmuch as they tear down trees and sometimes impede the drainage of a country and give rise to peat morasses. The term.
weathering" includes all the superficial changes which rocks undergo in conse quence Of atmospheric action. Everywhere disintegration is going on more or less rapidly: Of all the terrestrial agents by which the surface of the earth is geologically modified, by far the most important is water. This substance exists in three forms; 1, vapor, invisible; 2, liquid, or water; 3. solid, as ice. By the sun's heat vast quantities of vapor are continually raised from the surface of the seas, rivers, lakes, snow-fields, and glaciers of the world. This vapor remains invisible until the air containing it is cooled down to below its dew-point, or point of saturation. At first, minute particles appear, which either remain in the liquid condition, or, if the temperature he sufficiently low, are frozen into ice. As these changes spread over a considerable area of sky they give rise to the phenomena of clouds. Further condensation augments the size of the cloud particles, and at last they fall to the earth, if liquid, as rain, if solid, as snow or hail. On the higher elevations they fall in snow, • and form glaciers, which send down their drainage to the valleys and plains. Much of the rairi sinks into the ground to gush out again in while the remainder pours down the slopes of the land, feeding brooks and torrents, which, swollen further by the springs, unite in rivers through which the drainage of the land is carried to the sea. From the sea the vapor again rises, to re-appear
in clouds and showers and to feed the streams of the land. Here is a vast system of circulation in perpetual renewal. And in all the system there is not a drbp of water which is not busy with its allotted task of changing the face of the earth. When the vapor ascends into the air it is almost chemically pure. But when, after being con densed into visible form, and working its way over or under the surface of the land, it once more enters the sea, it is no longer pure, but more or less loaded with material taken by it out of the air, rocks, or soils through which it has traveled. Day by day the process is advancing. So far as we know it has never ceased since the first shower fell upon the earth. We may well believe, therefore, that it must have worked marvels upon the surface of the planet in past time, and that it may effect vast transformations in the future.
Under the head of TERRESTRIAL WATERS, we must consider rain, underground water, brooks, rivers, lakes, frost, river ice. snow, hail, and glaciers. Rain produces two changes on the surface: it acts chemically on soils and stones, and sinking into the ground, continues a series of similar reactions there. It also acts mechanically by wash ing away loose materials, and thus powerfully affecting the contours of the land. Rain eontains carbonic acid absorbed from the air, and some other ingredients, in addition to its natural hydrogen and oxygen. Rain water contains on the average 2 pet' et. of gas which is composed of 66.4 nitrogen, 31.2 oxygen, and 2.4 carbonic acid. Common salt, ammonia, sulphates, nitric acid, inorganic dust, and organic matter are usually present in minute quantities in rain water. The ingredients chiefly effective in chemical reactions are oxygen, carbonic acid, and organic matter. The effect of water upon rocks and other solid matter scarcely needs explanation. It is always more or less in the direction of decomposition, There is probably no known substance which is not, under some condition, soluble in containing carbonic .aqi4 .cir other natural re-agents. As rain is so universally distributed over the globe, this chemical action must be of very general occurrence. The usual results of the fall of rain upon a land surface must be a disintegration and consequent lowering of that surface. To form a true conception of this action we need to watch what takes place over a wide region. The whole land surface over which rain falls is exposed to waste. The superficial covering of decayed rock or soil is constantly, though slowly, traveling downward to the sea. In this ceaseless' transport rain acts as the great common carrier. The par ticles of rock loosened by the atmospheric waste, by frost, or by the_ chemical action of the rain itself, are washed off to form a new soil. But they, as well as the particles of the soil, are, step by step, moved downward over the face of the land till they reach the nearest brook or river, whence their seaward progress may be rapid. A heavy rain' dis colors the water-courses of a country, because it loads them with the fine debris which it removes from the general surface of the land. In this way rain serves as the means whereby the work of the other disintegrating forces is made conducive to the general degradation of the land. The decomposed crust produced by weathering, which would otherwise accumulate over the solid rock and protect it from further decay, is removed by rain so as to expose a fresh surface to further decomposition. This decay is general and constant, but not uniform. In some places, from the nature of the rock, from the flatness of the,ground, or from other causes, rain works under great difficulties. There the rate of waste must consequently be extremely slow. In other places, again, the rate may be rapid enough to be appreciable from year to year. A survey of this department of geological activity shows how the unequal wasting by rain has helped to produce the details of the present condition of the land; those tracts where the de struction has been greatest, forming hollows and valleys, others, where it has been' less. rising into ridges and hills. Rain-action is not always merely destructive. Usually it is accompanied by reproductive effects, and, as already remarked, the moldered rock which it washes off furnishes materials for the formation of soil. In favorable situations it has gathered together accumulations of loam and earth from neighboring higher ground—the " brick-earth." " head," and " rain-wash " of the south of England— earthy deposits, sometimes full of angular stones, derived from the suhaerial waste of neighboring rocks.