WATER 545 



Distilled water can be aerated at the same time by a Normandy's 

 condenser. On British warships five gallons per man per diem is 

 distilled and aerated. 



In land districts where there are no rivers, wells, or storage tanks 

 for the rainfall, boring should be carried out. This should be done 

 in a hillv country for preference. The required depth will depend 

 upon the distance from the surface of the impervious layer. 



Select the following places for experimental borings : — 



(i) Under dry river beds. 



(2) At the junction of ravines and valleys. 



(3) At the foot of hills. 



(4) In a thickly w^ooded country. When a country is cleared of 

 vegetation and timber, the rain water flows away very rapidly and 

 permanent streams are few. 



(5) Examine along the line of any geological fault in the part. 



(6) Test localized areas over \vhich fogs or swarms of insects are 

 noticed in the early morning. 



(7) Limestone and sandstone districts are good for deep wells, but 

 unfortunately there are large inland areas where such do not exist. 



We will now deal with the subject under three heads, viz. : Rain 

 Water; Surface Water; Subterranean Water. 



Rain Water. 



Rain water is soft, well aerated and pure, apart from the impurities 

 taken up by it from the atmosphere. It also absorbs atmospheric 

 gases, such as carbon dioxide, oxygen and nitrogen, the last two in 

 the proportion of 8 and i6'5 per litre. 



Tropical rain water varies much in quantity, from 600 inches per 

 annum at Cherrapunji, in India, to rainless desert tracts. At Aden 

 rain may not fall for several years. 



Rain water may be collected from the roofs and stored in tanks, or 

 from large surface areas made impermeable by cement, &c., and stored 

 as at Gibraltar. When collected from iron roofs the latter should not 

 be painted, as the paint, containing lead, may lead to lead poisoning. 



In some parts, as in the Choco, Colombia, there is nearly one inch 

 of rainfall every night all the year round, so that permanent storage is 

 not required and fresh w-ater is always available. 



Birds should ahvays be kept off collecting roofs as they pollute the 

 water. Wires should be placed above and along the ridges and the 

 guttering to keep them off. 



In rainy districts pollution from the dust and decayed vegetation 

 does not arise. 



To determine the rain water available it is necessary to know two 

 things : — 



