68 ELEMENTARY SCIENCE 



count of their limited knowledge of iron-working. The 

 High Bridge over the Harlem River in New York City was 

 built to serve as an aqueduct as well as a bridge. 



The pipes which convey water through city streets are 

 called mains. The smaller pipes which enter the houses 

 are called service-pipes. Water-meters are installed in the 

 houses to record the amount of water used. The point 

 at which a service-pipe goes through the foundation wall 

 of a house is often the point of greatest danger from freez- 

 ing. So it is desirable to have the service-pipe enter the 

 house on the sunny side. It is also desirable to have the 

 pipes protected by wrapping where they are most exposed 

 to cold. The depth at which service-pipes must be laid 

 to prevent bursting from freezing depends on the climate 

 and the nature of the soil. Loose, gravelly soil freezes 

 much deeper than more compact soil. 



In low and level country stand-pipes are commonly used 

 in order to give gravity flow. The water is pumped up 

 into them. But in cities with high ground near them, the 

 water is pumped into high-lying reservoirs instead. 



When the water comes from a river, or from some other 

 source of doubtful purity, filters are used to remove the 

 impurities. A filter is a construction of porous material 

 through which the water is strained. Sand, gravel, char- 

 coal, and porcelain are substances used for filters. Even 

 filtered water, however, may contain disease germs, and 

 when there is any doubt about its purity, it should be 

 boiled. This destroys the germs. 



