ize of this spring reservoir depends on the average rate of flow
of the spring and on the quantity of water used. If there is always an
overflow from the spring, that is, if it always at all times of the year
furnishes more water than is required by the house at that time of day
when the greatest demand is made, then a two-foot sewer pipe is just as
good as a concrete chamber ten feet square. But if at times the spring
is low, so that the flow during the night must be saved to compensate
for the excess consumption during the day, or if the rate at which the
water is drawn at certain hours is greater than the average rate at
which the spring flows, then storage must be allowed for in preparing
the spring to act as a reservoir.

We have already estimated that a family of ten persons might use five
hundred gallons of water a day, and the most exacting conditions would
never require the spring to hold more than one day's supply. This would
mean a chamber four feet deep and in area four by five feet. If the
average supply of the spring is less than the average consumption of the
family, then the spring must become a storage basin for the purpose of
carrying water enough over the dry season, and the capacity of the basin
must be computed from the number of days' storage required. It may not
be out of place to suggest again the possibility of increasing the yield
of the spring by laying draintile in a ditch running along the permeable
stratum. These pipes may run fifty or one hundred feet each way from the
main spring, so long as they continue to find ground water.

The walls of such a spring reservoir as here suggested for depths of six
to eight feet need not be more than nine inches thick, whether built of
brick or concrete. For greater depths the thickness should be increased
to twelve inches.

[Illustration: FIG. 38.--A protected spring-chamber.]

The roof of the spring-chamber may be of plank, but this is temporary
and undesirable. It is far better, for all spans up to ten feet, to make
the roof a flat slab of concrete six inches thick, imbedding in the
concrete in the bottom of the mass some one-half-inch iron rods, spaced
about a foot apart each way and extending well into the side walls. The
size of these rods should increase with the size of the chamber, making
them three-quarter-inch rods up to a nine-foot span, and one-inch rods
up to a twelve-foot span. There should be some way of getting into the
spring, prefer