It is unquestionably of the utmost importance that wells be carefully covered over, and every precaution should be taken to prevent surface-water leaking into them around their edges. In order to comply with these conditions a pump is essential, since it is the only means by which water can be brought to the surface without exposing the contents of the well to contamination. It is likewise of the first importance to have the walls of the well curbed to a sufficient depth to prevent the possibility of seepage from the surface. It is, of course, also quite necessary that the well be of sufficient depth--the lower we go the more likely are we to secure a perfectly pure water. In regions where the water rises to within eight or ten feet, or less, of the surface, the possibility of the well being contaminated during the rainy season by seepage is considerably increased, and the waters of such wells should be used only after analyses have shown that they are pure; where this cannot be done, the water should be boiled before being drunk. Of course, the possibilities of contamination are greatly increased if the locality be thickly inhabited. As has been before remarked, cisterns are more liable to contamination from the air than are wells, chiefly owing to the fact that they are supplied by water that is conducted into them by gutters from the tops of houses. There is no question that during the dry seasons dust containing many kinds of bacteria is deposited all over the tops of houses and remains there until washed away by the rains. While it is true that the sunlight quickly kills most germs that produce disease a certain number of them would inevitably escape, and having gained entrance to a cistern, would be likely to multiply and later cause trouble. It is thus seen that however pure the rain-water may originally have been--and it is among the purest of all waters--it is likely to become contaminated in the process of collection, and may ultimately in this way become the source of disease. Where any doubt exists as to the purity of such water it should be boiled before use.