Water-mill in Golden era:
The water-mills were also invented in the classical period. But they came to be widely used only in the Middle Ages. The wind-mills and water-mills harnessed nature for performing mechanical work. These mills were used for grinding grains, extracting oil from seeds and drawing water from wells, thus helping agriculture. They were also used for blowing bellows, forging iron or sawing wood. Mills became so popular that a mill and a miller were found in every lord's domain. The task of making and servicing the wind and water-mills was beyond the skill of most village smiths. Therefore, there grew a trade of mill-wrights who went about the country, making and mending mills. These men were the first mechanics who knew all about the making and working of gears. They also had a hand in the development of mechanical clocks and watches. There were two navigational invent~ons, the compass and the sternpost rudder, that had a profound impact on sea voyages in the Middle Ages. The earlier sea trade routes were along the coastline of various countries. With these two inventions, the oceans were thrown open to trade, exploration, and even war for the first time. Open-sea navigation quired accurate charts of the position of stars, latitudes etc. and gave an impetus to later developments in astronomy and geography. It also raised the urgent problem of finding the longitude. The need for compasses and other navigation instruments brought into being a new skilled industry.