32 
On Water Supplies suited to 
monia and air, and on the roofs it acquires a yellow, and even 
inky colour by dissolving a good deal of organic matter, and 
slowly washing away the excrement of birds and cats, as well as 
the nests and dead bodies of birds and insects, which are fre- 
quently found in the gutters. These impurities form a fitting 
food for infusoria ; hence any germs of disease, floating in the 
air or resting on the roofs, find a congenial home where they 
can breed and multiply. In addition, a good deal of soot is 
washed down, which, at times, gives the water a very offensive 
colour and smell. For all these reasons, filtration through sand, 
at all events, is indispensable ; but a perfectly safe, colourless, 
and almost tasteless water can be produced by purification with 
iron. I say almost tasteless, because it is very difficult, if not 
impossible, wholly to remove the sooty taste and smell, so that 
it will not be perceived in drinking-water. 
When it is desired to use the rainfall as a source of supply, it 
is indispensable to have tanks or reservoirs large enough to store 
every drop of water that falls. From a study of the records 
of the monthly rainfall in various places in England, I find 
that the storage capacity has to be from 15 per cent, to 20 per 
cent, of the total rainfall. If monthly records, extending over 
some years, have been kept in the neighbourhood of a proposed 
water supply, they should be examined, and the most irregular 
year picked out. The mean monthly rainfall, usually given in 
terms of the depth of water fallen in inches, will be found by 
dividing the total fall by 12 ; then all the rain which fell con- 
secutively above the mean during a number of months will 
represent the number of inches of storage required. 
Thus in Manchester, during the year 1858, 36*48 inches of 
rain fell, which is at the rate of 3*04 inches per month; this 
beintr also the average rate at which the water may be con- 
sumed. During the month of August there fell 1"43 inches 
over the mean, during September, 0'69 inches, and during 
October, 2 "16 inches ; making a total of 4*28 inches above the 
mean consumption. There was no other period during that 
vear when the excess was so large ; hence the storage required 
would be 4 '28 inches deep out of a total fall of 36 "48 inches, 
which is nearly 12 per cent. It would generally be safe to store 
one-sixth of the total rainfall. 
The quantity of rain which falls every year varies enormously, 
according to the position of the locality. The western side of 
the country has three times as much rain as the east coast, and 
the mountainous districts are more abundantly watered than the 
plains. I will take 24 inches per annum, or 2 inches a month, 
as a safe average. 
The quantity o'f water required per head depends on the 
