Dutch rivers and harbours constantly silt up as a result of sand carried in the water building up on the riverbed or harbour floor. Regular dredging is required to prevent waterways from becoming too shallow. Around 1800 much of this dredging was done by horse-driven mills. These ‘horse mills’ drove a chain with dippers that scooped up the mud and silt. On the front deck of this model are the stalls for the horses.