It usually made fifty, and sometimes sixty miles a day.

The rate depended on the number of locks it had to pass through.
Probably most of my young readers understand the nature of a lock.

As all water seeks a level, there would be danger in an uneven country that some parts of the canal would be left entirely dry, and in others the water would overflow.

For this reason at intervals locks are constructed, composed of brief sections of the canal barricaded at each end by gates.

When a boat is going down, the near gates are thrown open and the boat enters the lock, the water rushing in till a level is secured; then the upper gates are closed, fastening the boat in the lock.