546 TROPICAL HYGIENE, SANITATION, ETC. 



The annual rainfall in inches as ascertained by a rain gauge. 



The sc|uare inches of sectional area of the building (not the slant 

 surface of the roof). 



The annual rainfall in inches multiplied by the square inches of 

 sectional catchment areas equals the cubic inches of water per annum. 

 This multiplied again by 0*0036 gives the number of gallons per 

 annum. 



The storage of rain water is often a problem. In Venice the system 

 is good, e.g., the soil is excavated, and the reservoir thus formed is 

 lined with brick and puddled clay. In the centre of this a brick well 

 is built, with openings in the sides towards the bottom of the shaft. 

 The remaining part of the reservoir is filled with sand, into which the 

 rain water first flows. In other countries there are underground tanks 

 (Buenos Aires), stone vaults (Jerusalem), or large galvanized vats on 

 pillars (Demerara and South Africa). Probably the best is a tank of 

 galvanized iron or in brick and cement, raised from the ground, pro- 

 tected bv a roof and made mosquito proof. All such, whatever their 

 construction, must be under the constant observation of a Medical 

 Officer of Health to control fly breeding, &c. 



When preparing storage tanks the longest duration of the dry 

 season must be known with the accurate rain returns and the amount 

 of water required daily. The average during twenty years, less one- 

 third, is about the rainfall in the driest year, and plus one-third in the 

 wettest year. 



Onlv a little more than one-half of the rainfall should be considered 

 available for storage. 



First washings from the roof should be rejected. This should be 

 by a small receiving tank with a float and plunger which, when the 

 tank is full, diverts the flow into the storage tank. A simple method 

 is to have the elbow of the tank supply pipe half-way between the 

 roof and the waste gully at the bottom. When the rain has flushed 

 the roof, plug the bottom of the pipe, the water then rises up to the 

 elbow, when it passes down into the tank. Some suggest that it is 

 always better to pass the water through a sand filter before running it 

 into the storage tank. 



All pipes should be protected by mosquito copper gauze. 



To ascertain the dimensions of supply pipes needed and the possible 

 discharge of cubic feet in a given time through a given pipe is often 

 troublesome. The following formula will be a guide : — 



If water with a head of H feet flows through L feet of pipe D inches 

 in diameter, the discharge W will be in cubic feet per minute : — 



W, = 472 / H X D' hence D = -538 -^ /JI^^ 

 ^' L '^ H 



