236 ECONOMIC GEOLOGY 
3-18 per cent. In the Queensland wells the decrease is from 
3 to 3% per cent. per annum. This decrease in yield has 
been accompanied in some areas by an increase in the cor- 
rosive quality of the water, and in some cases by a rise of 
temperature, in one well amounting to 10° F. in two years. 
Hence in some parts of the basin the proportion of the plutonic 
water has increased, showing that some of it is still rising 
into the water-bearing beds. 
Town SuppLiEs AND SETTLEMENT—Early settlements are 
usually limited to areas where there is an available supply 
of water either from a river or lake or from shallow wells in 
sand or gravel. Clay in thick beds is only available for 
residence after a water supply is provided by pipes or aque- 
duct. London, for example, drew its water from the Thames 
nearby, until the supply became too impure for domestic 
use. The suburbs were limited to areas of gravel, for wells 
in London Clay had to go through it into the underlying sands 
and chalk, and were too deep and costly for single dwellings. 
Cities depend for their water either on deep wells or on 
supplies brought from outside either from rivers or lakes. 
London now draws its main supply from the Thames above 
the area affected by the tide. In summer the flow of the 
Thames over the lowest lock at T eddington is often only 
200 million gallons a day, and has been as low as 154 million 
(August, 1887). Hence as London needs an average from 
the Thames of 230 million gallons a day, it would require 
sometimes more than the total flow; and the flow in the 
Thames must not be reduced by ‘withdrawal to less than 
100 million gallons a day. Hence if London depended 
on a supply drawn daily from the Thames it would often 
be thirsty. Winter floods discharge as much as 7500 million 
gallons a day. Supplies are then collected in immense 
storage reservoirs, which are available for use throughout 
the year. This system has the advantage that the water is 
improved and purified by storage; noxious germs are de- 
stroyed, and the water is rendered innocuous. 
Cities near mountainous country draw supplies from lakes 
or artificial reservoirs, for the rainfall is usually high and 
the run-off carries a high proportion into the reservoirs. 
Glasgow was the pioneer of this system by, in 1859, drawing 
its supply from Loch Katrine: Manchester obtains its water