Water Supply and Drainage 



241 



horizontal area rather than much depth, owing to the fact that the gutters must 

 be above the height of its extreme capacity. This limit of capacity should be 

 established about 6 inches below the top of the cistern, and an overflow made at 

 that height to avoid any chance damage to ceilings and the like. This overflow 

 can lead to a cellar cistern if desired. It is safer to have the cistern set on a 

 zinc base, which shall project beyond it on all sides and be turned up several 

 inches and connected with a waste pipe. This will save considerable if the 

 cistern should leak. The tank is usually made of 2-inch planking well bolted 

 together. The average rainfall in the Northern states is about 48 inches per 

 year; in the whole country, about 36 per year. Considering that a roof of 1,000 

 square feet would (in Philadelphia) receive an average of 74 gallons per day, 

 that 8 gallons per capita should be allowed, and that one gallon contains 231 cubic 

 inches, it is easy to get some idea as to the size of the cistern required. If 

 sanitary appliances are to be used, the amount required would be fully twice the 

 ordinary, if not more. It is necessary that the tank be cleaned out every three 

 months or so. This is important. 



Water is carried from the source of supply by two methods, gravity and 

 pumping. When the source is higher than the required height to which water is to 

 be brought, the problem is easily solved by gravity. In case the supply source is 

 considerable, the water can be forced to a higher tank by use of the "ram." This 

 simple mechanical 

 device, which op- 

 erates automatically, 

 enables one to raise 

 a small quantity of 

 water by utilising 

 the force of a large 

 body of water. It 

 takes advantage of 

 the impulse of a flow 

 suddenly checked 

 and directed into the 

 desired channel. Its 

 working is easily ex- 

 plained (see Fig. 44). 

 Water entering the 

 machine through the 

 pipe (a) flows to the 



limit of the DiDe and 

 r , 11 



finds an outlet above 



the valve (c), which, when not under pressure, falls of its own weight and thus 

 allows the escape. The water continues to flow until its momentum is sufficient 

 to raise the valve (c), thus closing the outlet. The flow, thus checked, rebounds 

 and finds another outlet through the valve in the air chamber (d) until the valve 

 (c) drops owing to a lack of pressure. The same performance is then repeated 

 and becomes continuous. The sudden impulse of water into the said air chamber 



P w der house which was originally a A water tower at Roxbury, Mass. Evidently 

 windmill. Somerville, Mass. influenced by the Colonial example 



