In protecting the spring and in building up around it in order to put it underground, concrete is the most suitable material, although a large sewer pipe or a heavy cask or barrel will answer the purpose. It is usually sufficient to dig out the spring to a depth of four or five feet, and with a pump it is possible to keep the water down, so that the concrete walls may be laid. In building these walls, it is important to notice from which side the spring water comes, and on that side holes should be left in the wall. These openings may properly be connected with agricultural tile drains laid out from the spring in different directions, serving both to drain the ground and to add volume to the spring. It is often possible instead of pumping out water during construction to drain a spring temporarily, in places where the ground slopes rapidly, by carrying out a drainpipe from the lowest level; this drain is to be later stopped up. The size 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.]