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An Encyclopedia of the History of Technology part 90

An Encyclopedia of the History of Technology part 90. This one of a kind encyclopedia presents the entire field of technology from rudimentary agricultural tools to communication satellites in this first of its kind reference source. Following an introduction that discusses basic tools, devices, and mechanisms, the chapters are grouped into five parts that provide detailed information on materials, power and engineering, transportation, communication and calculation, and technology and society, revealing how different technologies have together evolved to produce enormous changes in the course of history | PART FIVE TECHNOLOGY AND SOCIETY Buckingham Palace and St George s Chapel Windsor. Coade stone under its later owners continued to be used until in late Victorian times the fashion for polychromy ousted it from favour. THE ARCH An arch is a structure composed of wedge-shaped blocks which are called voussoirs and designed to span a void. The blocks support each other by mutual pressure and therefore capable of withstanding considerable weight and thrust. A wooden framework centering was set up underneath the arch to support it until all voussoirs were in position. Parts of an arch Often the central voussoir at the top of an arch is larger and made into an architectural feature this is then called a keystone. The point or line where the arch springs up from its supporting wall or column is known as the springing line. The lowest voussoir on each side of the arch is thus known as a springer. The support wall pier or column is the abutment. The span of an arch is the distance between abutments and the height in the centre is the rise. The outer curve of the line of voussoirs is called the extrados the inner is the intrados the under surface is the soffit. These parts and the different forms of arch are shown in Figure 18.3. History of building with arches Knowledge of arcuated construction that is building structurally with arches rather than in the post and lintel manner see p. 857 was gained by most of the early cultures but in some of these notably the Egyptian and the Greek plentiful supplies of stone and marble were available so trabeated structures were possible. Moreover the builders held the opinion strongly that the arch was a less stable form of structure. Indeed the Greeks had a proverb which said the arch never sleeps referring to the truism that if the arch support were to be weakened the ever-present outward thrust of the curve of the arch would cause collapse. So in both Egypt and Greece arcuated structures were confined to small and utilitarian .