THE AIR. Through this interchange of gases the Air surrounds us on all sides, although air is kept pure for man and animals. we cannot see it. Mankind, animals, and plants depend upon it for the "breath of life." If there were no air, or atmosphere, surrounding every part of the earth, there could be no living thing upon it. Air is made up of several gases and vapor of water. We will only speak of the two more important gases and the vapor of water, because our lives depend more upon these. The principal life-giving gas is OXYGEN. It is given to the air from all plant life. People and animals would suffocate if it were not for the oxygen which they breathe in, and which keeps their blood pure. Oxygen causes the burning of all substances. It also causes the rusting of iron and the decay of many substances. There is always a greater or les amount of VAPOR OF WATER in the air which is also needed by all living things How does the moisture get into THE ATMOSPHERE SURROUNDS THE EARTH. CARBONIC-ACID GAS is given to the air by all burning and decaying substances, and in the breath of all animals. It would poison all animal life and mankind if allowed to remain in the air. A wise means was provided by our Creator for taking this poison from the air. Carbonic-acid gas is needed by all plant life. All vegetation breathes it in. The plants could not live without it. the air? The streets and all sur faces are said to "dry off" after a rain, but the moisture has not all soaked into the ground. It has simply disap peared. It has beet absorbed into the air. It has bee changed from wate which is easily seen, to vapor, which is unseen, by the process of evaporatio The mud-puddles will dry faster on a warm day that up on a cool day. This is because warm air will hold more moisture than cold ai The puddles will dry faster on a windy day than on a still day. On which day will more air come in contact with the surface of the water? Water is evaporating all over the earth, giving to the air the needed moisture to support life. Water evaporates faster in the warm regions than in the cold regions, although snow is always evaporating. How does the water form to make rain! Breathe on a cold mirror or glass. It is misty, in a room of ordinary warmth. Can you think whence the tiny drops Plants cool more quickly than the air at night. The air which immediately surrounds them becomes cool and deposits dew. Frost instead of dew is deposited when we are having freezing weather. Whenever warm air filled with moisture comes in contact with cold air the vapor is changed. It is changed from vapor, which we cannot see, to a light, fleecy substance called mist, which we can see. This is the process of condensation. When this light, fleecy substance lies or hangs near the ground it is called fog. If it is high up in the sky it is called a cloud. When it is freezing weather the moisture in the clouds forms into snow. A DRY REGION FOR MOST OF THE YEAR. The main river of a system not only cuts its own channel in the mountain side or in the plateau, but each tributary stream does exactly the same thing. Study all of the pictures. Trace the tributary basins. Describe each. All streams have a work to do in cutting down the highlands, wearing out the val leys, and in carrying and depositing mate The deposit is a delta. Find the deltas in the rial to build up the plain or to build out the pictures. seashore. Some rivers flow in the val leys between the mountain folds. Other riv ers must be older than the mountains themselves. their banks and spread out like great lakes on the level land, submerging They flowed across the everything in their way. This is a flood. fold, and as the mountain slowly arose The land covered at flood-time is a FLOOD-PLAIN. The water which covers the flood-plain is filled with fine soil or with sand. When the flood subsides and the stream returns to its banks, the flood-plain is covered with mud. For a long time the water stands upon the surface in ponds. The floods cause most flood-plains to have a very rich, moist soil. During the flood the valley sides are widened and cut back nearer to the source of the stream. THE HUDSON RIVER, NEW YORK. above the plain, the river cut it down, thus gapping the mountain ridge. RIVERS AND RIVER BASINS. The Delaware gaps the Appalachian Mountains. The Potomac gaps the mountains at the place where you see the rapids in the picture. Can you tell from the picture on page 84 in which direction the Potomac is flowing? The moun tains must have been a long, long time in forming, to have given the river time to cut through. There are rapids usually in a river where it cuts across the highlands. They are formed by the water flowing over the rocky ridge which has not yet been cut to the bed level of the river. 85 cause of the rapidly sloping land. In the flood-plains, which are nearly level in many places, it loses the power given If you build up a highland and make a plain on the sand table or in the schoolyard, and sprinkle it with water, you will see that the water will cut its way directly down the highland by the shortest way to the lower land. On the plain it will work A CITY ON A FLOOD-PLAIN, GERMANY. by the slope; so it flows in and out in long, swinging curves. Trace these curves or ox-bows in the pictures. A MINING TOWN BUILT ON THE FLOOD PLAIN OF A RIVER IN THE ROCKY MOUNTAINS. its way down the slight slope, making many curves. In the same way a river cuts a more direct course among the highlands be If you observe a carriage or a car go around a curve you will see that the wheels on the outer curve turn around many more times than those on the inner curve. This is because both wheels must get around the curve in the same time. The water in the stream flows much faster on the outer curve than on the inner. This causes the water which is moving so swiftly to cut away the bank on the outer curve, while it helps the slowly moving water on the inner curve to deposit the soil which it carries, as in the picture of a mud plain on page 83. Rivers often cut deep gorges in the rocky surface where there are few feeders to widen the valley. Describe the gorge in the picture on page 83. In countries where the weather is usually dry the river valleys are not widened as rapidly as in and rapids. This enables boats to ha a continuous line of transportation a lessens the expense of shipping. Ho Rivers hel the lumberme How? The waterfalls in our rivers have long bee utilized by man for turning mill wheels. More and more, how ever, steam is taking the place of water power alone. The tremendous force of Niagara a used in generating electricity to light and serve as motor power in cities the higher land of the valley slopes. Great cities or trade-centers are upon the rivers. These are connected by railways and boats, which serve as outlets for the products of the great farming districts. Canals are usually made around falls long distance from the fall. The Niagara Falls a Fall on the left of the picture is about 165 fe and the Canadian Fall is about 150 feet high. one of the wonders of the world. The America Carefully study and describe each of the pictur on this page. Observe how the water is weari down the rocky ledge in the Yosemite Falls. |