The real value of the judgment of the water cannot be secured until a careful examination of the sources of supply has been made. This examination ought to include the whole watershed from which the supply comes. Otherwise, there will be a defect in the data which cannot be easily supplied. In addition to this, also the geological structure of the rocks and the character of the soil are of importance, since often, especially in limestone regions, due to the subterranean passages, water supplies may be contaminated from a great distance, without the contour of the soil leading anyone to suspect that such a contamination would take place.