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The great canal of Darius I: 6th century BC

The cutting of canals for irrigation has been an essential part of the civilization of Mesopotamia, controlling the water of the Euphrates and the Tigris. Several canals link the two rivers, and small boats use these waterways. But the world's first canal created purely for water transport is an incomparably more ambitious affair.

Between about 520 and 510 BC the Persian emperor, Darius I, invests heavily in the economy of his newly conquered province of Egypt. He builds a canal linking the Nile and the Red Sea. Its access to the sea is close to modern Ismailia, which much later becomes the terminus of another great waterway, the Suez canal.

Roman roads: 2nd century BC - 2nd century AD

The great network of Roman roads, the arterial system of the empire, is constructed largely by the soldiers of the legions, often with the assistance of prisoners of war or slave labour. The amount of labour involved is vast, for these highways are elaborate technological undertakings.

The average width of a Roman road is about 10 yards. Below the paved surface the fabric extends to a depth of 4 or 5 feet in a succession of carefully constructed layers.

First a trench is excavated. Its bottom is rammed hard, and if necessary is strengthened by driving in piles. Then four successive layers are constructed, each a foot or more thick. The first is of masonry, laid in cement or clay. Above this is a course of concrete, then gravel and cement. Finally the top layer is laid in dressed stones, sloping away in a pronounced camber from the centre.

The designers of the Roman roads are single-minded. Paying scant attention to the demands of contours, and having few property rights to consider, their mission is to drive the road straight ahead. The legions will march far in the empire, but they will take the shortest route.

Part of the purpose of the Roman roads is speed of communication, so there are posthouses with fresh horses every 10 miles along the route and lodgings for travellers every 25 miles. By the 2nd century AD the network spreads all round the Mediterranean and throughout Europe up to the Danube, the Rhine and northern England, amounting in all to some 50,000 miles. This far outdoes even the very impressive achievement of the Persian roads . Travellers on foot or horseback have rarely been so well provided for.

For haulage purposes these roads are less satisfactory, because the straight line results in some very steep hills. Anyone with a wagon and horse would prefer an attitude less severe than that of the Roman road engineer.

The Grand Canal: 3rd century BC - 13th century AD

The Chinese (the greatest early builders of canals) undertake several major projects from the 3rd century BC onwards. These waterways combine the functions of irrigation and transport.

Over the centuries more and more such canals are constructed. Finally, in the Sui dynasty (7th century AD), vast armies of labourers are marshalled for the task of joining many existing waterways into the famous Grand Canal. Barges can now travel all the way from the Yangtze to the Yellow River, and then on up the Wei to the western capital at Xi'an.

Along this great Chinese thoroughfare the rice harvest of the Yangtze is conveyed to the centres of political power in the north.

From the 13th century there is a new northern capital. Kublai Khan establishes himself at Beijing, which becomes the capital of the Mongol or Yüan dynasty. The Mongols extend the Grand Canal all the way north to join Beijing's river at T'ien-ching.

Flash locks and pound locks: 10th - 15th century

From the very first construction of canals, some method is necessary to cope with differences in water level. The simplest solution is a weir, to hold up the water on the higher side, with a gap in the middle which can be opened to let a boat through. The removal of the barrier, however achieved, is inevitably followed by a sudden rush of water - carrying the vessel easily through in one direction, but making passage very difficult in the other. A primitive lock of this kind is known, for obvious reasons, as a flash lock.

The development of the more sophisiticated pound lock is traditionally credited to an engineer, Chiao Wei-yo, working on the great Chinese canal system in the 10th century AD.

It is said that Chiao is required to construct two flash locks on the Grand Canal only about 200 yards apart. He realizes that he has created a pool which will be at the upper or lower level of the canal depending on which of the two barriers is open. Moreover the barrier separating patches of level water can be opened without the obstruction of water pressure.

The result is the pound lock, standard on all modern canals. The first in Europe is believed to have been built in the Netherlands in 1373 at Vreeswijk, where a canal from Utrecht joins the river Lek.

At this stage the barrier is a simple sluicegate which has to be raised and lowered like a guillotine. The process is laborious, and the water pressure against the flat surface requires a very strong construction to hold it.

The last missing piece in the design of the modern lock is the mitred lock gate. On this system each end of the lock is closed by a pair of wooden gates slightly too large to close in a normal flush position. They meet with mitred edges pointing in the direction of the higher water level. Water pressure holds them tightly together, until the level is the same on either side - at which point the gates can be easily pushed open.

The first lock with mitred gates is probably the one built in Milan in about 1500 to join two canals of differing levels. Known as the San Marco lock, it is likely that its design is by Leonardo da Vinci. As his notebooks reveal, Leonardo is interested in all aspects of hydraulic engineering; and he is employed at this time by the duke of Milan.

From the 12th century Europeans have been busy constructing canals, even with the primitive device of the flash lock. The mitre lock makes possible increasingly ambitious projects.

Junks and caravels: 12th - 15th century

In both east and west the centuries known in Europe as the late Middle Ages and early Renaissance see vast improvements in long-distance travel by sea. China is the pioneer. While Europeans are making ocean journeys in long narrow ships with a single square sail (the longships of the Vikings), the Chinese are improving the design of the junk.

From the 12th century junks grow in size, strengthened now by bulkheads. Soon they are steered and stabilized by an important innovation, the sternpost rudder. And they begin to be powered by sails on multiple masts.

These improvements make possible the expeditions carried out between 1405 and 1433 by the eunuch admiral Zheng He. He even reaches the east coast of Africa, a decade or two before the Portuguese exploration down the west coast of the continent.

The Portuguese successes depend, like the Chinese, on improvements in the design and construction of ships. The caravel is much smaller than the junk, but it is better suited to sailing in violent oceans. With the caravel, travel becomes possible to any coast in the world other than the frozen Arctic and Antarctic. A caravel takes Magellan's crew on the first circumnavigation of the globe in 1519-22.

Inca roads: 15th century

The Inca roads, the arteries of an empire, amount in all to more than 14,000 miles. They are not paved, in the way of Roman roads, nor are they even much flattened - for this empire contains no wheeled vehicle nor any horses.

The Incas rule over massively varied terrain, made up of large areas of jungle, desert and rugged highlands. Their roads are in effect paths, kept clear in these difficult conditions. Suspension bridges span small ravines, enabling runners to hurry unimpeded with a message - or caravans of llamas to make slower but steady progress with bales of raw materials and precious fabrics.

European canals: 12th - 17th century

In one area of Europe, the Netherlands, canal building is an integral part of economic development. The primary purpose is drainage; an efficient transport network is a welcome bonus. But in Italy, in the late 12th century, an ambitious canal is constructed without any subsidiary motive of drainage or even irrigation.

It is the Naviglio Grande, built between 1179 and 1209 to bring marble from near Lake Maggiore for the construction of the cathedral in Milan. The barges float down the river Ticino before diverting into the canal, which has a fall of 110 feet in its length of 31 miles. The next comparable project, a century later, is a canal with a different purpose - to improve trade.

From 1391 the Stecknitz canal is constructed southwards from the city of Lübeck. Its destination is the Elbe, which is reached early in the 15th century. The new waterway joins the Baltic to the North Sea.

This canal rises some 40 feet from Lübeck to the region of Möllner and then falls the same amount again to reach the Elbe, all in a distance of 36 miles. This must be about the limit which can be safely achieved with flash locks. With mitre locks, from the 16th century, anything is possible. And the most ambitious projects are undertaken in France.

The Briare canal, completed in 1642, joins the Seine to the Loire; at one point it has a staircase of six consecutive locks to cope with a descent of 65 feet over a short distance. Even more remarkable is the Canal du Midi, completed in 1681, which joins the Mediterranean to the Atlantic by means of 150 miles of man-made waterway linking the Aude and Garonne rivers. At one point this canal descends 206 feet in 32 miles; three aqueducts are constructed to carry it over rivers; a tunnel 180 yards long pierces through one patch of high ground.

The potential of canals is self-evident. It falls to Britain, in the next century, to construct the first integrated system of waterborne traffic.

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