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exercised when applying such wells on a large scale as in the 
supply of towns. 
A second source of supply is that from rivers.— Where it is possible 
to obtain a supply from rivers, absolutely free from the drainage 
of towns, influx of chemical works and other causes of impurity, 
no supply possesses so many advantages. 
The water passing into rivers does not all filter through the 
ground, a considerable portion rapidly flowing over the surface 
and down the slopes of the hills ; this being the case, river water 
is generally chemically purer than that of springs, as it has not 
the same opportunity for taking up the mineral impurities present 
in the ground. aaa 
River water being also exposed to the air, parts with some of 
the excess of carbonic acid, and this tends to soften the water. 
Some authorities have great faith in the power of running 
streams in removing organic impurities; there is, however, 
considerable difference of opinion on this point. 
The objections to river water are the occasional absence of 
limpidity, particularly during floods. The waters deposited by 
Tain carry with them a certain amount of detritus and other 
foreign matter derived from the soil; this proportion is sometimes 
very considerable, and entails great expense in arrangements for 
filtering. 
The water from rivers also varies much in temperature during 
the year, having a range of more than 30°; this would be a 
considerable drawback if the water supplied to our houses arrived 
at the same temperature as at the source. 
That the temperature of water has an influence on health has 
been shewn by various authorities, the deaths from diarrhcea in 
districts supplied from rivers having been proved to be higher 
than in those districts supplied from springs of which the variation 
at the different seasons of the year is but slight ; and when the 
temperature of the water reaches 62° the disease becomes 
_ epidemic. 
