Farms and Villages. 
43. 
stroyed by the slow percolation through vast masses of strata, 
which exert a beneficial influence, partly by mechanical and 
partly by chemical action. One considerable disadvantage, 
however, is that the water is often hard. The next best source 
is, generally, the water of rivers, which, although liable to 
contamination from many sources, such as the drainage of 
cultivated fields, of houses, villages, and towns, have yet a 
marvellous power of self-purification. A stream considerably 
polluted will lose all trace of contamination in a run of two or 
three miles. Scientific men are not agreed as to the methods 
by which this purification is accomplished. It is probably due 
to several causes, namely, to the large surface constantly exposed 
to the action of the oxygen of the air, as the particles of water 
roll over each other ; to the effect produced by aquatic plants 
and animals in assimilating the obnoxious, but to them, nutri- 
tive substances ; and lastly, to the action of microbes, causing 
decomposition and precipitation of substances held in solution. 
Of course these natural purifying agents may be, and often are 
overpowered by extreme pollution, and there is always danger 
of zymotic contagion from the drain of a single house dis- 
charging near the point where a supply is taken. 
Lakes or ponds without outlets are objectionable, because all 
the polluted matter carried in by the surrounding water-shed 
remains and becomes concentrated ; but where outlets exist, 
they form very excellent sources of supply, especially if drawn 
upon at some distance from the shore, out of the reach of the 
immediate surface drainage. 
Springs and rivers are sometimes so situated, that they are at 
a higher altitude than the places to be supplied, and in such 
cases, if the source has a minimum flow sufficient for the needs 
of the premises, nothing need be done but to construct a small 
basin to collect the spring, or to place a lateral sluice and 
strainer below the lowest summer level for the purpose of 
tapping the river or the lake. But very commonly springs an^ 
rivers fail towards the autumn ; it then becomes necessary to 
construct reservoirs of sufficient capacity to store up two or 
three months' supply, and this may become an expensive matter. 
In the case of rivers, they are very commonly converted into 
reservoirs by placing weirs across them and collecting the water 
in the reaches above. Such a proceeding is not always prac- 
ticable, on account of riparian rights and of the large area of 
good land which may be submerged. Again, most rivers, in 
flood time, have their waters greatly discoloured and rendered 
very muddy ; in such cases, storage reservoirs are desirable,, 
for they can be filled when the water is in good condition, and 
drawn upon in time of flood. But very commonly springs, , 
