THE PUBLIC WATER, MILK, AND FOOD SUPPLY 349 



below this a layer of coarse sand and then layers of gravel, 

 pebbles, and stones. Pipes are placed beneath to carry off 

 the purified water. The filter beds are flooded with water, 

 which soaks through and conies out below purified. Not 

 only have the larger particles been filtered out, but what is 

 of much greater importance, the water has been freed of 

 bacteria and decaying organic material. 



Strange as it may seem, the work of clearing the foul 

 water of the injurious bacteria is done by other bacteria that 

 are beneficial, which live 

 on the small grains of sand 

 and attack and destroy the 

 harmful bacteria as they 

 pass through the filter. In 

 order that these bacteria 

 may work to best advan- 



FIG. 143. Cross-section of a sand filter. 



tage, they must have the 

 oxygen found in the air, so 

 that if the water is very 

 foul, it is not allowed to 

 cover the filter beds all the time. But it is occasionally drained 

 off so that the bed may be exposed and the bacteria given a 

 chance to get oxygen in the air. The sand filter operates 

 slowly and therefore many of the large cities now use rapid 

 filter beds with alum and hypochlorite. Following are two 

 instances in which large municipalities have found means 

 of reducing the death rate by securing, under adverse con- 

 ditions, a supply of pure water. 



Cleveland, Ohio. Cleveland took its drinking water from 

 Lake Erie and formerly emptied its sewage into the same 

 lake not far from where the water was taken in. As the city 

 grew larger, the number of cases of typhoid fever increased, 

 till finally such an epidemic broke out that people became 

 frightened and the city authorities saw that something must 

 be done. Accordingly experts were called in to study the 



