Wind is simple air in motion. It is caused by the uneven heating of the earth’s surface by the sun. Since the earth’s surface is made of very different types of land and water, it absorbs the sun’s heat at different rates.A windmill is a machine that is powered by the energy of the wind. It is designed to convert the energy of the wind into more useful forms using rotating blades or sails. The term also refers to the structure it is commonly built on. In much of Europe, windmills served originally to grind grain, though later applications included pumping water and, more recently, generation of electricity. Recent electricity generating versions are referred to as wind turbines.We can't talk about wind mill without mention it's country origin Holland.Windmills have always played a great part in the life of Holland and its inhabitants. While at first they served to grind corn, to remove excess water from the low-lying districts, and to saw timber, thus making the country fit for human habitation and adding to habitable area, they developed - especially in the seventeenth century - into a most important factor in the social structure of those days. It is with increasing interest that one learns about this.It is only in Holland that so many windmills are present in so small an area. These windmills moreover are in very reasonable, many of them even in excellent, condition and a considerable number of them are working regularly. There are windmills of the most varied types: drainage mills, corn mills, and industrial mills for all sorts of purposes electricities mills but the last one cost more than normal electricities plant.Water pumping windmills and small wind electric generators were once also used throughout the United States. Rural electrification programs of the 1930’s and 1940’s largely replaced the need for these systems. Starting in the early 1980’s, Federal and State Government policies and incentives led to a revival in wind power generation.Efficiency is important for windmills, because the entire cost is in the technology, not the source of energy, which is wind.The most significant factor determining the efficiency of windmills is the number of blades. This is because more blades capture more energy. Large windmills must use few blades due to stress on the metal. When the diameter of rotation is 300 ft (100 m), two blade systems must be used. When the diameter is 150 ft, three blade systems can be used.Also, metal stress becomes the limiting factor with large size. Smaller windmills reduce that problem. One of the most significant facts about size is that steel gets like rubber when large. Excessive steel and heavy mass create the overriding expenses for large windmills.The government experimenters said they could produce electricity at the usual price by using their large two blade systems, which required wind speeds in excess of 15 mph. This means that if systems were designed to use lower velocity winds with less turbulence, they would have produced electricity cheaper than other sources.