PART IV 
ENGINEERING GEOLOGY 
CHAPTER XIX 
WATER SUPPLY 
FresH water is one of the most essential of human needs. 
The people on some South Sea Islands have no means of 
satching rain, the wells are brackish, coco-nut milk is the 
usual drink, and, according to Admiral Wharton, they enjoy 
the luxury of fresh water only when it can be skimmed off 
the lagoons after heavy rain. As a rule fresh water is a 
primary need, and as people become more fastidious as to 
its quality and extravagant as to quantity, competition is 
keen for the unappropriated supplies. 
Tue THREE SourcEs oF WaTER—METEORIC—Most fresh 
water is provided by rain and is therefore said to be meteoric. 
Rain is mainly due to evaporation from the sea. The mois- 
ture in the air is condensed and either falls as rain or is 
deposited on cool surfaces as dew. The pre-historic dew- 
ponds of the South of England were attributed to dew— 
“Only the dew-pond on the height, unfed, which never 
fails "—but they are fed by rain-water which is protected 
from percolation and evaporation by the structure of the 
pond (E. A. Martin, Dew-ponds, 1915). The average annual 
rainfall of the British Isles is estimated by Dr. H. R. Mill 
at about 40 inches; and as I inch of rain provides 22,622 
gallons per acre; 40 inches on an acre amounts to 900,000 
gallons. The area of the British Isles being 77,683,084 
acres, and the population about 44 million, the rain supplies 
each inhabitant with 14 million gallons a year, or 4000 gallons 
a day. 
Rain, as a product of distillation, might be expected to 
He chemically pure; but it washes from the air dust, dirt, 
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