ank must be large enough to tide over these
intervals of no rainfall. In the temperate zone there is no regularity
in the monthly rates of rainfall. In the eastern part of the United
States, the months of June and September are usually the months of least
precipitation, although the general impression, perhaps, is that July
and August have less rainfall than any other months. The truth is that,
while wells and rivers are low in July and August, the actual rainfall
for those months is not below the normal, and the low flows in the
streams are caused by excessive evaporation and by the demands of
growing crops. Although June and September have usually less rainfall
than other months, in Boston the fall has been as high as 8.01 inches in
June and 11.95 inches in September. Again, in Boston, typifying the
eastern part of the United States, and taken because of the great length
of rainfall statistics available there, the two months of highest
rainfall on the average are March and August, and yet, in each month, in
some particular year, the rainfall has been the lowest for any of the
twelve months in the year.

As shown by statistics, the average rainfall in each month, taking a
period of forty years or so, is practically constant for each month, and
it is only the deviations from the average which would make trouble in a
supply tank depending upon rainfall. Fortunately, statistics also show
that while a month whose average rate of rainfall is three inches may be
as low as three tenths of an inch, it is not often that two months of
minimum rainfall come together, and in looking over the rainfall
statistics the writer finds that for any three consecutive months,
including the minimum, the amount of rainfall is generally two thirds of
the monthly average for that year; and this is stated in this way
because it gives what seems to the writer a basis for determining a fair
and reasonable capacity of a rain-water storage tank. It depends, one
will notice, on the average annual rainfall; that is, on the depth to
which the rainfall would reach in any year if none ran off. This varies
from about ten inches in the southeastern part of the United States to
one hundred inches in the extreme northwest, the average for the eastern
part of the country being about forty-five inches, so that the monthly
average is 3.75 inches.

_Computation for rain-water storage._

With this for a basis, it may be determined how large a storage tank
ough