through low wet lands or over certain kinds of clayey soils, surface drainage is not all that is necessary. Common side drains catch surface water and surface water only. Isaac Potter says:
"Many miles of road are on low, flat lands and on springy soils, and thousands of miles of prairie roads are, for many weeks in the year, laid on a wet subsoil. In all such cases, and, indeed, in every case where the nature of the ground is not such as to insure quick drainage, the road may be vastly benefited by under drainage. An under drain clears the soil of surplus water, dries it, warms it, and makes impossible the formation of deep, heavy, frozen crusts, which are found in every undrained road when the severe winter weather follows the heavy fall rains. This crust causes nine-tenths of the difficulties of travel in the time of sudden or long-continued thaws.
"Roads constructed over wet undrained lands are always difficult to manage and expensive to maintain, and they are liable to be broken up in wet weather or after frosts. It will be much cheaper in the
