252 FIRST YEAR COURSE IN GENERAL SCIENCE 



carries. In 1873 the city of Vienna was supplied with 

 water from the Alps, seventy miles away. That was one of 

 the first enterprises of the kind. Water is now carried a 

 much greater distance from the mountains to Los Angeles, 

 San Francisco, and other cities. 



Natural water is not chemically pure, but for domestic 

 uses it should be clean. It is made clean by filtering it 

 through beds of sand and gravel constructed between the 

 receiving or storage reservoir and the distributing reservoir. 

 The filtering removes some minute organisms and sedi- 

 ment. As disease germs are too small to be removed by 

 filtering, they must be prevented from entering the 

 reservoir by keeping the drainage basin clear of all 

 dwellings, camps, and cattle. State laws give to cities 

 and towns the right to remove all buildings from land which 

 has been selected for a basin to furnish a water supply. 

 Suitable compensation, of course, is made to the owners. 



In the region of the great central plain, where in many 

 towns the people depend on wells for water supply, the 

 same care should be taken to prevent pollution of the 

 water. Many large cities use river or lake water which 

 has been exposed to contamination. In such cases, chemi- 

 cal means are sometimes used to destroy germs and purify 

 the water. (LABORATORY MANUAL, Exercise XXIII.) 



EXERCISES 



1. Describe the beginning of a glacier. 



2. Where is the glacier that is nearest to your home? 



3. Are there glaciers on mountains in your state? Why? 



4. What change of conditions of the earth's surface or of the 

 atmosphere might cause a glacier to form in the Allegheny Mountains? 



6. How do people know that a glacier once covered North America 

 as far south as the Ohio River? 



6. Where are there now great glaciers in North America? 



7. Moving at an average rate of 20 in. a day and melting 22 in., 

 how would the end of a glacier change position from April 1 to 

 October 1? '* 



