Whereas the engines in propeller-powered aircraft generally achieve their maximum efficiency at much lower speeds and altitudes, jet engines and aircraft achieve maximum efficiency (see specific impulse) at speeds close to or even well above the speed of sound. Jet aircraft generally cruise at faster than about M 0.8 (609mph, 981km/h or 273m/s) at altitudes around 10,000–15,000 metres (33,000–49,000ft) or more.
Frank Whittle, an English inventor and RAF officer, developed the concept of the jet engine in 1928,Hans von Ohain in Germany, a decade later developed the concept independently at the end of the 1930s. He wrote in February 1936 to Ernst Heinkel, telling him of the design and its possibilities. However, it can be argued that the English engineer A. A. Griffith, who published a paper in July 1926 on compressors and turbines, also deserves credit.
Great Zimbabwe is a ruined city in the south-eastern hills of Zimbabwe near Lake Mutirikwe and the town of Masvingo. It was the capital of the Kingdom of Zimbabwe during the country's Late Iron Age. Construction on the monument by ancestors of the Shona people began in the 11th century and continued until the 15th century, spanning an area of 722 hectares (1,780 acres) which, at its peak, could have housed up to 18,000 people. It is recognised as a World Heritage Site by UNESCO.
Great Zimbabwe served as a royal palace for the Zimbabwean monarch and would have been used as the seat of political power. One of its most prominent features were the walls, some of which were over five metres high and which were constructed without mortar. Eventually the city was abandoned and fell into ruin.
The earliest known written mention of the ruins was in 1531 by Vicente Pegado, captain of the Portuguese garrison of Sofala, who recorded it as Symbaoe. The first European visit may have been made by the Portuguese traveler António Fernandes in 1513-1515, who crossed twice and reported in detail the region of present-day Zimbabwe (including the Shona kingdoms) and also fortified centers in stone without mortar. However, passing en route a few miles north and about 35 miles south of the site, he did not make a reference to the Great Zimbabwe riddle.