3. Do lakes and ponds generally freeze to the bottom? Would they be more, or less, likely to, if ice were heavier than water? 4. How is salt commonly obtained from salt water? 5. Do you suppose any substances besides water act as solvents? What is the solvent used in most medicines? 6. When the water used for cooking potatoes has begun boiling, will the potatoes be cooked more quickly, if there are two burners instead of one under the boiling water? 7. Is any dessert made by freezing flavored water instead of cream? 8. What causes the bursting of water pipes in winter? Why are hydrants sometimes left turned on a little during a very cold night? 9. Why is it easier to swim in the ocean than in fresh water? 10. How does the freezing of the ground help to break it up for the next year's crops? CHAPTER XV WATER SUPPLY AND SEWERAGE 131. Why Do We Need So Much Water? Have you ever thought how much we depend upon an abundant supply of water? In our homes we need it for bathing, for cooking and for drinking, for washing dishes and clothing, for spraying lawns and gardens and to carry waste material into the sewerage system. As ice it preserves our food and with salt makes our common freezing mixture. Industries need water to furnish steam power for machinery. Some industries, like refineries for petroleum and sugar, as well as laundries, tanneries, slaughter houses, starch factories, gas works, paper mills, and dye works, need it for washing and rinsing on a large scale. Cities need water for fire protection and for cleaning streets and to carry away sewage. 132. How Do Cities Get Their Water? — Have you ever seen the waterworks of your community? It is necessary that water shall have some pressure in the pipes, so that it will flow from the faucets rapidly and will rise to the top floors of buildings. There must also be enough pressure so that we can throw a powerful stream of water in case of fire. Some cities get their water to flow by a gravity" system. This means that the water level of some pure stream, reservoir, or lake near the city is higher than any buildings of the city, so the water can simply flow downhill to the city and up into its buildings. Denver has such a water supply. If a gravity system is not possible, water is often pumped by a strong steam force pump up into a high tank, or reservoir, or "standpipe"; from this it can flow down to the ground and up into buildings. You have seen pictures of the great Roman aqueducts, by which water was brought to the city of Rome from distant lakes or springs. New York City now receives its water through a great aqueduct from the Catskills, 90 miles away. At one place the water is carried under the Hudson River. Los Angeles gets its water from mountains many miles away. The cities on the Great Lakes get their water supply from the lakes. As the sewage from the city may pollute the water, the "intakes" (places where the water is taken in) are usually several miles from the shore. Chicago has built the Drainage Canal " at great cost, to carry water from Lake Michigan into the Illinois River; thus it sends the city's sewage into the Mississippi instead of letting it pollute Lake Michigan. 66 66 133. How Do We Get Water in the Country? — In the country we get water from cisterns, wells, or springs. We dig wells, not to strike underground streams or "veins" of water, but to make holes into which the groundwater" can run. This is the water which saturates the ground everywhere below certain levels. You can see how this is, if you dig a hole into ground that is water-soaked after a rain. Deep wells are not dug, but are drilled, that is, bored through soil and rock until a level is reached at which good water in sufficient amounts enters the well. The deeper a well, the greater is the area of the ground from which its water will be collected and the more likely it will be to last through a dry season. Artesian wells are deep wells drilled through a layer of rock or clay which holds the water at great pressure. When the covering is pierced, the pressure of the water forces the stream to the top of the well and sometimes high into the air. Springs have a different origin. When water soaks into the ground until it reaches a layer of rock or clay that it cannot penetrate very FIG. 84. Water soaks through the porous sand until it reaches the compact clay which it cannot penetrate. At the bottom of the hill a spring is formed. easily, it will collect above this layer. If the ground is sloping and the rock layer comes near the surface, as on a hillside, the water bubbles out as a spring (Fig. 84). 66 134. What is Plumbing? - How much does your family pay for water? In some cities you pay a flat rate," that is, no matter how much you use, you pay a certain sum. In other places a meter is put into your house to measure the volume of water which flows from the street main into your plumbing system. "Plumbing comes from the Latin word for "lead"; the |