Canal lock gates, 19th century.
© Science Museum / Science & Society Picture Library
Coloured plan. The first locks on canals in Britain date from Elizabethan times, and the design of locks has changed little since then. A boat enters a chamber, and a pair of gates is shut behind it. The chamber is then emptied (if the boat is descending) or allowed to fill with water (if ascending) until the water level is the same as that in the canal in the direction in which the boat is travelling. Once this is the case, another pair of gates in front of the boat are opened, and the boat can continue its journey. Canal builders tried to find routes requiring as few locks as posible as they slowed traffic considerably. Canal building peaked in Britain in the early 19th century, but by the 1840s, the railways had begun to take trade away from the canals.