Mills

Mills: Harvesting Water & Wind Power 

Water Mills

The ancient Egyptians, Greeks and Romans knew that the force of current in a river could be turned into mechanical energy by the use of a water wheel. Egyptians used a wheel of paddles, set in a river, to pump water up from the river for irrigation. Between the 3rd and 1st centuries BCE, Hellenistic engineers discovered that water wheels could be used to pump water out of mine shafts. The Romans used water power for pumping, and occasionally for grinding grains into flour. By the 3rd century CE, they had developed turbines (water screws with angled blades, built into water-tunnels to force the current through them at faster speeds).

However, the use of water power in the Roman Empire was surprisingly limited. The cost of building and maintaining water mills exceeded the cost of slave labor to accomplish the same tasks, and slaves were not restricted to locations with fast-moving water. Consequently, water power only came to the forefront in the medieval period.

Innovation upon the Roman concept of the water wheel began in Ireland, where the first tide-powered mill appeared in the early 600's. The tide mill captures the stored power of an incoming tide, which fills a man-made reservoir (usually built from a natural inlet on the coast) that is then closed off automatically by a one-way gate to keep the water as the tide recedes. Once the tide retreats low enough, the stored water is released through a flume, which the Irish altered to be a variable flow-gate.

The limitation of the tide mill is that it could only be used at certain times of the day, dependent upon the tide cycle, and it had to be built along the coastline. However, a tide mill is extremely consistent and predictable (unaffected by drought, rainstorms or other variables which vex traditional water mills), and coast lines were plentiful in Ireland, England, France and Spain, where they spread throughout the early medieval period. Because the flume controlled the amount of flow directed at the wheel or turbine, the miller could deliberately vary the speed of rotation without needing geared power controls. The majority of these mills were built of local stone and materials, and continued in operation for centuries with minimal upkeep.

The mill-race and gated flume also radically altered the practicality of upright and horizontal water wheels inland from the coasts, along streams and rivers. Most pre-medieval water mills relied on the direct power of the river current, with the sole exception of a few turbine channels. Medieval Europeans discovered that water wheels could be mounted underneath falling water at the end of an artificial waterfall (making an “overshot” wheel), thus harnessing both the lateral speed of the current and the falling speed of the water as gravity pulled it down onto the wheel. This made for a more powerful watermill, and added incentive for building a mill race.

Medieval communities also began building dams across streams and rivers, with mill “chases” to direct the controlled flow of water from the dam to a mill. This allowed them to store up river power and build their mills in more secure, flood-proof locations away from the riverside. Furthermore, dams could help slow down rainwater runoff and prevent flood damage downriver, and could provide water reserves that served to buffer against droughts. With so many benefits, the dams and water mills went viral; by 1086, William the Conqueror's Domesday Book reported 5,624 water mills in England alone (roughly one water mill for every fifty families; but this is an undercount, given the incomplete nature of the Norman census) (1).

As Europe moved away from dependence upon slave power, the utility of the water mill became ever more apparent. Water power could turn a shaft directly, operating a massive millstone that made quick work of grinding grain. However, the shaft could also be combined with gears and cams to provide hammering power and reciprocating power. This in turn led to the developments of hammer mills (as for fulling wool textiles) and reciprocating sawmills (for cutting wood and stone), in addition the use of mills to turn grindstones for sharpening tools, fans for forge bellows, and presses for pulping crops (as for cider or syrup). The massive labor-saving power of each single mill made the cost of building them obviously worthwhile to communities across Europe.

Wind Mills

The ancient Greeks discovered that sails, such as those from ships, could be mounted to rotating frames and the wind would turn them. Intrigued by this, they designed a few types of vertical windmills and understood that they could be implemented to turn shafts. However, both they and the Romans seem to have abandoned the concept there, rather than harnessing this power to perform daily tasks.

The oldest known examples of functional windmills come from the early medieval period, and are located in Persia. The sails, woven of wattle hurdles, were mounted along an upright post that could turn horizontally (around a vertical axis), built high up into adobe walls that formed a wind tunnel. Below the wind tunnel, the rotating shafts reached down to turn millstones mounted under them. This was a very direct, efficient method of harnessing wind, and it continued to be used (some of these windmills are still in existence today).

Medieval Europeans, however, could not count on the wind always coming from one direction, and so these early horizontal mills with wind tunnels did not spread into Europe until the 12th century. When windmills were finally adopted, it was with a radical change in orientation and design (so much so that historians debate whether they were invented independently, rather than adapted from the Eastern style).

These new windmills spun vertically (around a horizontal axis), with the rotating axle mounted atop a post and using gears to harness the power for millstones below. The post itself was designed to rotate in its base, so that the sails could face into the wind no matter which direction it came from. These windmills were so successful at harnessing power that they rapidly populated the whole European countryside (to the point that there were lawsuits against neighbors who blocked the wind).

Windmills could be placed anywhere, not just alongside flowing streams and rivers. The base of a windmill could be made of any local materials, so the only timber beams needed were for the post and axle. Once built, maintaining a windmill required less labor than maintaining a water mill—and the maintenance of aboveground, dry structures is safer than working to repoint or repair underwater walls.

Windmills enabled communities to tackle impossible new tasks, such as continuously pumping water from a well into an elevated cistern for irrigation, far from flowing water sources, or even pumping underwater lands dry and enclosing them with dikes. Many of the fertile “lowland” areas of Northern Europe were thus literally reclaimed from the sea by the power of windmills. Anything that could be done by water power could be done by wind power, but with fewer limitations on location and materials, less risk of damage by floods, and easier, safer maintenance as a whole.

Sources:

1) Gies, Joseph, and Francis Gies. Leonard of Pisa and the New Mathematics of the Middle Ages. Crowell, 1969. Page 113.