Metals seldom occur in their pure form in nature. They occur mostly in the form of ores, which means they are mixed with rocks. Through the process of smelting, the metal is then melted ‘out’ of the rocky mass, for instance, iron from iron ore. Even in 6000 BC there were furnaces for smelting copper ore. Metals are used mainly in the form of alloys which is a mixture of two metals. For instance, iron is often changed into steel by adding carbon. Depending upon how much carbon is mixed, the steel is either soft and can be moulded or is hard and can be used in making knife blades.
Diamonds are very expensive because they occur at relatively few places in the world – like Russia and South Africa – and are very difficult to extract. They are made of carbon and are the hardest substance known to us, which is the result of the high pressure at their place of origin deep below the Earth’s surface. They are naturally thrown up on the Earth’s surface through volcanic eruptions. For this reason they are mined in the vents of the extinct volcanoes. Being very hard they have great industrial use for grinding tools. Diamonds can also be made artificially by creating the same conditions as found under the surface of the Earth, but these artificially made diamonds are not as valuable as natural diamonds. 
Windmills are machines that are driven by wind. Their blades have slightly diagonal surfaces. If the wind hits them from front, they deflect it to the side and in this way convert the wind pressure in a rotary movement. Initially, millers had to set the blades themselves in such a way that the wind came onto them from the front. The rotary movement of the blades gets transferred to the milling stones inside the mill through cogwheels. Windmills were traditionally used to mill corn to make flour and grate spices. They also acted as drive for oil presses and sawmills. 
When compared with the two-blade rotors, the three-blade rotors are more stable and quieter. The disadvantage of the two-blade rotors is that they need a special ‘wobble’ motor to prevent the severe collision when the rotor passes the tower. When the rotor blade is on top, the wind presses against it with full force and bends it. At the same time, the opposite rotor blade is in front of the tower. Thus, it cannot counteract the high pressure at the upper blade which leads to a strong jerk which is compensated by the wobble motor. This problem does not occur in three-blade rotors because the blades are never exactly opposite to one another. In addition, the three blades form a kind of disc for the flowing wind and are hardly prone to faults. 
When we sail at ‘high wind’, we travel – as far as possible – against the wind. The wind, thus, comes diagonally from the front. The propulsion takes place when the wind sweeps quickly past the sails, which are possibly set up in the middle of the boat. In contrast, if the boat travels ‘before the wind’; the sails are diagonal to the boat. The wind comes from behind, blows into the sails, and pushes the boat forwards.