What is a canal? It is a trench cut in the ground, filled with water deep enough for a well-laden boat, and wide enough for boats to pass each other. On one bank is a path, called the towpath, upon which horses or mules travel, pulling a canal boat behind them by means of a long rope. In most canals it is found necessary to lift the boats over higher land or up to a higher level. This is done by locks, which are built where the two levels of the canal come together. These locks are shut off from each part of the canal by gates. When the lower gates are shut and the upper gates open, water is let into the lock from the upper canal until on a level with it. Then a canal boat from the upper canal enters the lock. The upper gates are closed, the lower gates opened, and the water runs out of the lock. The boat, remaining on top of the water, sinks to the lower level and is ready to proceed on its course. In traveling the other way the process is turned about. The boat enters the lock and rises with the water which is let in from above until it is on the upper level.
Canals, with their locks, are simple and easily built. The expense lies mainly in digging the trench. When the canal is once finished the cost of running is very slight, and freight can be carried much more cheaply than over roads, or even by the natural rivers. Canal travel is very slow, however, as the boat is drawn by a horse at a slow walk. Therefore a canal is used, for the most part, to carry freight, especially freight not very perishable. Garden vegetables, fruit, and meats, for example, are not carried on canals to any great distance; on the other hand, the length of time used in conveying salt, or flour, or household goods, is not of so much importance.
Plans for canals sprang up all at once throughout the country. The Middlesex canal in Massachusetts and the Blackstone canal between Providence and Worcester were among the first built. The Delaware and Hudson canal in New York, and the Chesapeake and Delaware in Maryland were of early importance. In time nearly every Atlantic State had one or more canals as aids to transportation. Many of them were of additional importance because they connected neighboring bays, and could furnish opportunities for water travel, even when the harbors might be blockaded in time of war.