As springs usually break out in a hollow or at the foot of a hill, unless carefully closed in they are quite liable to contamination from rain water from the surrounding surface of the ground.

Where springs of a sufficient size can be reached, or a sufficiently "live" series of deep wells can be bored, these furnish a safe source of water supply for cities.

But of course not more than one city in five or ten is so favored.
Mountain Reservoirs.

Two other methods of securing a water supply are now generally adopted.

One is to pick out some stream up in the hills or mountains, within fifteen miles or so of the city, and put in a dam, thus making a reservoir, or to enlarge some lake which already exists there.