It is an astonishing concept to the modern mind that medieval man was surrounded by machines. The fact is, machines were not something foreign or remote to the townsman or to the peasant in his fields. The most common was the mill, converting the power of water or wind into work: grinding corn, crushing olives, fulling cloth, tanning leather, making paper, and so on. These were the factories of the Middle Ages. In the towns and villages the citizen could stand on a bridge over a river or canal and observe the different types of water mill: mills built along the banks, others floating midstream or moored to the banks, and, if he cared to look under the bridge, he might find the same machines built between the arches. If he walked upstream he would find the river dammed to provide a sufficient fall of water to drive the mills' machinery.