ARCHIMEDES' PRINCIPLE 67 



cities. A surprising amount of water is needed for this 

 purpose. The daily consumption of water per person for 

 certain cities has been estimated in gallons as follows: 

 New York 79, Chicago 140, Philadelphia 132, St. Louis 72, 

 Boston 80, Washington 187. In European cities it is 

 much less, because of restrictions. The question of water- 

 supply is often one of the most serious questions which a 

 city has to face. 



The purity of water is just as important to a city as 

 its amount, for many of the commonest and most serious 

 diseases, especially typhoid fever, are caused by impure 

 water. In many cities the death-rate has been much 

 lowered by purifying the water-supply. Thus in Chicago 

 after the opening of the drainage canal, which purified 

 the supply of water from Lake Michigan, the rate of death 

 from typhoid alone was reduced by two-thirds. This 

 represents a saving of nearly a thousand lives a year. 



Ancient Rome had an elaborate system of water-works. 

 Portions of its aqueducts still stand; they were very excel- 

 lent constructions (aqua, water; duco, lead). These aque- 

 ducts were for the purpose of leading water across valleys 

 and lowlands from its source in the highlands. Such a 

 system is called a gravity system; it depends wholly upon 

 the force of gravity to secure the desired flow of water. 

 Gravity systems of water-works are largely used to-day. 

 However, it is not necessary to build high aqueducts across 

 valleys as it was in the days of ancient Rome. The water 

 may be carried across valleys and under rivers in iron 

 pipes, which rise and re-enter the aqueduct on the opposite 

 side. As long as the rise is not higher than the head of the 

 water-supply, the water will continue to flow, however deep 

 its plunge. The ancients could not use this plan on ac- 



