Working water

Sections of a massive iron chain, and several of the oak buckets attached to it, were discovered during a large-scale excavation at 30 Gresham Street in the City of London. Museum of London archaeologists found the remains in a deep well that had been dug in AD108-9. They were perfectly preserved by waterlogging. The well-house had been destroyed by fire. The machinery had fallen into the well and never been recovered.

By carefully measuring the components and the depth of the well, we could work out that there must have been 30 buckets in the chain. Each bucket contains 7 litres and weighs 7kg when empty, 14kg when full. The iron components for each bucket weigh 6kg. Therefore, the total weight of the bucket chain could have been as much as 500kg (half a ton).

From the shape of the chain links we could tell that the drive wheel must have been facetted (with flat sides). Experiment showed that it would have had a minimum of eight sides to work efficiently.

The buckets were made from oak planks, jointed together and nailed. The chain links were made from wrought iron. Roman wrought iron had a higher carbon and silicon content than modern wrought iron, making it less prone to corrosion. In the reconstruction we have used puddled iron - the nearest modern equivalent.

Descriptions given by the Romans themselves of similar types of machinery are helpful here. Initially we thought the wheel would have been powered directly by a treadwheel. One or two people would have had to step continuously on the outside of the wheel - rather like climbing an infinite flight of stairs!

However, the engineers soon calculated that the chain would have been too heavy to be powered by a treadwheel. In fact it requires 0.3 horsepower. So we have reconstructed the machine with a capstan that can be turned by people or animals. The operator walks nine metres for each revolution of the drive-wheel. The horizontal motion of the capstan is translated into the vertical motion of the chain through gears.

Engineers calculate that the original machine could have emptied water at the rate of two litres per second. The machine can therefore raise 120 litres (25 gallons) per minute; 7200 litres (1500 gallons) per hour; 72,000 litres (15,000 gallons) per ten-hour day.

Today, the daily water requirement for each person in the UK is around nine litres (two gallons). On that basis, the machine could easily have supplied at least 8000 people - perhaps one third of the estimated population of Londinium in the 2nd century AD.

Francis Grew & Jenny Hall
Department of Early London History and Collections, Museum of London
December 2002

Visit the Museum to see...

The reconstructed water lifting machine is on display in the Rotunda Garden. Some of the original excavated artefacts are also in a special display in the Museum foyer. These displays are for a limited period only. Please check before making a special visit.

On the Museum website...

Find out more about how the machine was reconstructed

Fact pack: Roman London: A brief history

The Museum also recently reconstructed a late Iron Age roundhouse which was based on evidence from a neighbouring excavation at 10 Gresham Street. Two Fact packs describe how the roundhouse was built and the archaeological background.