parallel engineering

see concurrent engineering

a team-based cooperative approach to product design and development, in which all parties are involved in new product development work in parallel. Concurrent engineering reduces or removes the time lag between the different stages of a product’s development, and earlier entry into a market is therefore possible. Product quality is improved, development and product costs are minimized, and competitiveness is increased.

Also known as parallel engineering, simultaneous engineering

SUBJECT AREA: Electricity, Electronics and information technology, Public utilities

[br]

[br]

[br]

After attending Owens College, Manchester, Hopkinson was admitted to Trinity College, Cambridge, in 1867 to read for the Mathematical Tripos. An appointment in 1872 with the lighthouse department of the Chance Optical Works in Birmingham directed his attention to electrical engineering. His most noteworthy contribution to lighthouse engineering was an optical system to produce flashing lights that distinguished between individual beacons. His extensive researches on the dielectric properties of glass were recognized when he was elected to a Fellowship of the Royal Society at the age of 29. Moving to London in 1877 he became established as a consulting engineer at a time when electricity supply was about to begin on a commercial scale. During the remainder of his life, Hopkinson's researches resulted in fundamental contributions to electrical engineering practice, dynamo design and alternating current machine theory. In making a critical study of the Edison dynamo he developed the principle of the magnetic circuit, a concept also arrived at by Gisbert Kapp around the same time. Hopkinson's improvement of the Edison dynamo by reducing the length of the field magnets almost doubled its output. In 1890, in addition to-his consulting practice, Hopkinson accepted a post as the first Professor of Electrical Engineering and Head of the Siemens laboratory recently established at King's College, London. Although he was not involved in lecturing, the position gave him the necessary facilities and staff and student assistance to continue his researches. Hopkinson was consulted on many proposals for electric traction and electricity supply, including schemes in London, Manchester, Liverpool and Leeds. He also advised Mather and Platt when they were acting as contractors for the locomotives and generating plant for the City and South London tube railway. As early as 1882 he considered that an ideal method of charging for the supply of electricity should be based on a two-part tariff, with a charge related to maximum demand together with a charge for energy supplied. Hopkinson was one the foremost expert witnesses of his day in patent actions and was himself the patentee of over forty inventions, of which the three-wire system of distribution and the series-parallel connection of traction motors were his most successful. Jointly with his brother Edward, John Hopkinson communicated the outcome of his investigations to the Royal Society in a paper entitled "Dynamo Electric Machinery" in 1886. In this he also described the later widely used "back to back" test for determining the characteristics of two identical machines. His interest in electrical machines led him to more fundamental research on magnetic materials, including the phenomenon of recalescence and the disappearance of magnetism at a well-defined temperature. For his work on the magnetic properties of iron, in 1890 he was awarded the Royal Society Royal Medal. He was a member of the Alpine Club and a pioneer of rock climbing in Britain; he died, together with three of his children, in a climbing accident.

[br]

SUBJECT AREA: Automotive engineering, Mechanical, pneumatic and hydraulic engineering

[br]

[br]

[br]

Ernest-Sylvain Bollée was the founder of an extensive dynasty of bellfounders based in Le Mans and in Orléans. He and his three sons, Amédée (1844–1917), Ernest-Sylvain fils (1846–1917) and Auguste (1847-?), were involved in work and patents on steam-and petrol-driven cars, on wind engines and on hydraulic rams. The presence of the Bollées' car industry in Le Mans was a factor in the establishment of the car races that are held there.

In 1868 Ernest-Sylvain Bollée père took out a patent for a wind engine, which at that time was well established in America and in England. In both these countries, variable-shuttered as well as fixed-blade wind engines were in production and patented, but the Ernest-Sylvain Bollée patent was for a type of wind engine that had not been seen before and is more akin to the water-driven turbine of the Jonval type, with its basic principle being parallel to the "rotor" and "stator". The wind drives through a fixed ring of blades on to a rotating ring that has a slightly greater number of blades. The blades of the fixed ring are curved in the opposite direction to those on the rotating blades and thus the air is directed onto the latter, causing it to rotate at a considerable speed: this is the "rotor". For greater efficiency a cuff of sheet iron can be attached to the "stator", giving a tunnel effect and driving more air at the "rotor". The head of this wind engine is turned to the wind by means of a wind-driven vane mounted in front of the blades. The wind vane adjusts the wind angle to enable the wind engine to run at a constant speed.

The fact that this wind engine was invented by the owner of a brass foundry, with all the gear trains between the wind vane and the head of the tower being of the highest-quality brass and, therefore, small in scale, lay behind its success. Also, it was of prefabricated construction, so that fixed lengths of cast-iron pillar were delivered, complete with twelve treads of cast-iron staircase fixed to the outside and wrought-iron stays. The drive from the wind engine was taken down the inside of the pillar to pumps at ground level.

Whilst the wind engines were being built for wealthy owners or communes, the work of the foundry continued. The three sons joined the family firm as partners and produced several steam-driven vehicles. These vehicles were the work of Amédée père and were l'Obéissante (1873); the Autobus (1880–3), of which some were built in Berlin under licence; the tram Bollée-Dalifol (1876); and the private car La Mancelle (1878). Another important line, in parallel with the pumping mechanism required for the wind engines, was the development of hydraulic rams, following the Montgolfier patent. In accordance with French practice, the firm was split three ways when Ernest-Sylvain Bollée père died. Amédée père inherited the car side of the business, but it is due to Amédée fils (1867– 1926) that the principal developments in car manufacture came into being. He developed the petrol-driven car after the impetus given by his grandfather, his father and his uncle Ernest-Sylvain fils. In 1887 he designed a four-stroke single-cylinder engine, although he also used engines designed by others such as Peugeot. He produced two luxurious saloon cars before putting Torpilleur on the road in 1898; this car competed in the Tour de France in 1899. Whilst designing other cars, Amédée's son Léon (1870–1913) developed the Voiturette, in 1896, and then began general manufacture of small cars on factory lines. The firm ceased work after a merger with the English firm of Morris in 1926. Auguste inherited the Eolienne or wind-engine side of the business; however, attracted to the artistic life, he sold out to Ernest Lebert in 1898 and settled in the Paris of the Impressionists. Lebert developed the wind-engine business and retained the basic "stator-rotor" form with a conventional lattice tower. He remained in Le Mans, carrying on the business of the manufacture of wind engines, pumps and hydraulic machinery, describing himself as a "Civil Engineer".

The hydraulic-ram business fell to Ernest-Sylvain fils and continued to thrive from a solid base of design and production. The foundry in Le Mans is still there but, more importantly, the bell foundry of Dominique Bollée in Saint-Jean-de-Braye in Orléans is still at work casting bells in the old way.

[br]

SUBJECT AREA: Electronics and information technology, Telecommunications

[br]

[br]

[br]

After early education at home and at private school, Eccles won a scholarship to the Royal College of Science (now Imperial College), London, where he gained a First Class BSc in physics in 1898. He then worked as a demonstrator at the college and studied coherers, for which he obtained a DSc in 1901. Increasingly interested in electrical engineering, he joined the Marconi Company in 1899 to work on oscillators at the Poole experimental radio station, but in 1904 he returned to academic life as Professor of Mathematics and Physics and Department Head at South West Polytechnic, Chelsea. There he discovered ways of using the negative resistance of galena-crystal detectors to generate oscillations and gave a mathematical description of the operation of the triode valve. In 1910 he became Reader in Engineering at University College, London, where he published a paper explaining the reflection of radio waves by the ionosphere and designed a 60 MHz short-wave transmitter. From 1916 to 1926 he was Professor of Applied Physics and Electrical Engineering at the Finsbury City \& Guilds College and a private consulting engineer. During the First World War he was a military scientific adviser and Secretary to the Joint Board of Scientific Societies. After the war he made many contributions to electronic-circuit development, many of them (including the Eccles-Jordan "flip-flop" patented in 1918 and used in binary counters) in conjunction with F.W.Jordan, about whom little seems to be known. Illness forced Eccles's premature academic retirement in 1926, but he remained active as a consultant for many years.

[br]

See also: Heaviside, Oliver; Kennelly, Arthur Edwin

SUBJECT AREA: Electronics and information technology

[br]

[br]

[br]

Born on a cattle ranch, Forrester obtained a BSc in electrical engineering at the University of Nebraska in 1939 and his MSc at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, where he remained to teach and carry out research. Becoming interested in computing, he established the Digital Computer Laboratory at MIT in 1945 and became involved in the construction of Whirlwind I, an early general-purpose computer completed in March 1951 and used for flight-simulation by the US Army Air Force. Finding the linear memories then available for storing data a major limiting factor in the speed at which computers were able to operate, he developed a three-dimensional store based on the binary switching of the state of small magnetic cores that could be addressed and switched by a matrix of wires carrying pulses of current. The machine used parallel synchronous fixed-point computing, with fifteen binary digits and a plus sign, i.e. 16 bits in all, and contained 5,000 vacuum tubes, eleven semiconductors and a 2 MHz clock for the arithmetic logic unit. It occupied a two-storey building and consumed 150kW of electricity. From his experience with the development and use of computers, he came to realize their great potential for the simulation and modelling of real situations and hence for the solution of a variety of management problems, using data communications and the technique now known as interactive graphics. His later career was therefore in this field, first at the MIT Lincoln Laboratory in Lexington, Massachusetts (1951) and subsequently (from 1956) as Professor at the Sloan School of Management at the Massachusetts Institute of Technology.

[br]

See also: Burks, Arthur Walter; Goldstine, Herman H.; Wilkes, Maurice Vincent; Williams, Sir Frederic Calland

SUBJECT AREA: Electricity

[br]

[br]

[br]

After completing a mechanical engineering apprenticeship Wilde commenced in business as a telegraph and lightning conductor specialist in Lancashire. Several years spent on the design of an alphabetic telegraph resulted in a number of patents. In 1864 he secured a patent for an electromagnetic generator which gave alternating current from a shuttle-wound armature, the field being excited by a small direct-current magneto. Wilde's invention was described to the Royal Society by Faraday in March 1866. When demonstrated at the Paris Exhibition of 1867, Wilde's machine produced sufficient power to maintain an arc light. The small size of the generator provided a contrast to the large and heavy magnetoelectric machines also exhibited. He discovered, by experiment, that alternators in synchronism could be connected in parallel. At about the same time John Hopkinson arrived at the same conclusions on theoretical grounds.

Between 1866 and 1877 he sold ninety-four machines with commutators for electroplating purposes, a number being purchased by Elkingtons of Birmingham. He also supplied generators for the first use of electric searchlights on battleships. In his early experiments Wilde was extremely close to the discovery of true self-excitation from remnant magnetism, a principle which he was to discover in 1867 on machines intended for electroplating. His patents proved to be financially successful and he retired from business in 1884. During the remaining thirty-five years of his life he published many scientific papers, turning from experimental work to philosophical and, finally, theological matters. His record as an inventor established him as a pioneer of electrical engineering, but his lack of scientific training was to restrict his later contributions.

[br]

SUBJECT AREA: Mechanical, pneumatic and hydraulic engineering, Photography, film and optics

[br]

[br]

[br]

As the son of a country gentleman, William Gascoigne would have had opportunities to receive reasonable schooling, but there is no record of how or where he was educated. However, by the late 1630s he had acquired a considerable knowledge of astronomy and was in correspondence with other scholars. About 1638 he invented an instrument to measure small angles in a telescope, consisting of two parallel wires in the eye piece moved by a calibrated screw. His invention remained unknown until it was reinvented thirty years later. He is said to have left the manuscript of a treatise on optics, but this did not survive. He was killed fighting for the royalist side at the battle of Marston Moor.

[br]

SUBJECT AREA: Electricity, Public utilities

[br]

[br]

[br]

Jablochkoff studied at the Military Engineering College in St Petersburg. Having a scientific bent, he was sent to the Military Galvano Technical School. At the end of his military service in 1871 he was appointed Director General of the Moscow-Kursk telegraph lines for the Midi Railway Company. At this time he began to develop an interest in electric lighting, and in 1875 he left the Imperial Telegraph Service to devote his time exclusively to scientific pursuits. He found employment at the workshop of M Bréguet in Paris, where Gramme dynamos and Serrin arc lamps were being constructed. After some experimentation he found a means of producing a carbon arc that regulated itself without any mechanism. This lamp, the Jablochkoff candle, with two carbon rods placed parallel to each other and so close that an arc formed at the ends, could continue to burn until the rods were consumed. Plaster of Paris was used to separate the two electrodes and crumbled away as the carbon burned, thus exposing fresh carbon. These lamps were used in May 1878 in Paris to illuminate the avenue de l'Opéra, and later in Rome and London, and in essence were the first practical electric street lighting. Since there was no regulating mechanism, several candles could be placed in a single circuit. Despite inherent defects, such as the inability to restart the lamps after they were extinguished by wind or interruption of supply, they remained in use for some purposes for several years on account of their simplicity and cheapness. In 1877 Jablochkoff obtained the earliest patent to employ transformers to distribute current in an alternating-current circuit.

[br]

SUBJECT AREA: Electronics and information technology, Public utilities, Telecommunications

[br]

[br]

[br]

After education at King's College, London, in 1852 Preece entered the office of Edwin Clark with the intention of becoming a civil engineer, but graduate studies at the Royal Institution under Faraday fired his enthusiasm for things electrical. His earliest work, as connected with telegraphy and in particular its application for securing the safe working of railways; in 1853 he obtained an appointment with the Electric and National Telegraph Company. In 1856 he became Superintendent of that company's southern district, but four years later he moved to telegraph work with the London and South West Railway. From 1858 to 1862 he was also Engineer to the Channel Islands Telegraph Company. When the various telegraph companies in Britain were transferred to the State in 1870, Preece became a Divisional Engineer in the General Post Office (GPO). Promotion followed in 1877, when he was appointed Chief Electrician to the Post Office. One of the first specimens of Bell's telephone was brought to England by Preece and exhibited at the British Association meeting in 1877. From 1892 to 1899 he served as Engineer-in-Chief to the Post Office. During this time he made a number of important contributions to telegraphy, including the use of water as part of telegraph circuits across the Solent (1882) and the Bristol Channel (1888). He also discovered the existence of inductive effects between parallel wires, and with Fleming showed that a current (thermionic) flowed between the hot filament and a cold conductor in an incandescent lamp.

Preece was distinguished by his administrative ability, some scientific insight, considerable engineering intuition and immense energy. He held erroneous views about telephone transmission and, not accepting the work of Oliver Heaviside, made many errors when planning trunk circuits. Prior to the successful use of Hertzian waves for wireless communication Preece carried out experiments, often on a large scale, in attempts at wireless communication by inductive methods. These became of historic interest only when the work of Maxwell and Hertz was developed by Guglielmo Marconi. It is to Preece that credit should be given for encouraging Marconi in 1896 and collaborating with him in his early experimental work on radio telegraphy.

While still employed by the Post Office, Preece contributed to the development of numerous early public electricity schemes, acting as Consultant and often supervising their construction. At Worcester he was responsible for Britain's largest nineteenth-century public hydro-electric station. He received a knighthood on his retirement in 1899, after which he continued his consulting practice in association with his two sons and Major Philip Cardew. Preece contributed some 136 papers and printed lectures to scientific journals, ninety-nine during the period 1877 to 1894.

[br]

GW / KF

SUBJECT AREA: Electricity, Land transport, Railways and locomotives

[br]

[br]

[br]

Davidson, son of an Aberdeen merchant, attended Marischal College, Aberdeen, between 1819 and 1822: his studies included mathematics, mechanics and chemistry. He subsequently joined his father's grocery business, which from time to time received enquiries for yeast: to meet these, Davidson began to manufacture yeast for sale and from that start built up a successful chemical manufacturing business with the emphasis on yeast and dyes. About 1837 he started to experiment first with electric batteries and then with motors. He invented a form of electromagnetic engine in which soft iron bars arranged on the periphery of a wooden cylinder, parallel to its axis, around which the cylinder could rotate, were attracted by fixed electromagnets. These were energized in turn by current controlled by a simple commutaring device. Electric current was produced by his batteries. His activities were brought to the attention of Michael Faraday and to the scientific world in general by a letter from Professor Forbes of King's College, Aberdeen. Davidson declined to patent his inventions, believing that all should be able freely to draw advantage from them, and in order to afford an opportunity for all interested parties to inspect them an exhibition was held at 36 Union Street, Aberdeen, in October 1840 to demonstrate his "apparatus actuated by electro-magnetic power". It included: a model locomotive carriage, large enough to carry two people, that ran on a railway; a turning lathe with tools for visitors to use; and a small printing machine. In the spring of 1842 he put on a similar exhibition in Edinburgh, this time including a sawmill. Davidson sought support from railway companies for further experiments and the construction of an electromagnetic locomotive; the Edinburgh exhibition successfully attracted the attention of the proprietors of the Edinburgh 585\& Glasgow Railway (E \& GR), whose line had been opened in February 1842. Davidson built a full-size locomotive incorporating his principle, apparently at the expense of the railway company. The locomotive weighed 7 tons: each of its two axles carried a cylinder upon which were fastened three iron bars, and four electromagnets were arranged in pairs on each side of the cylinders. The motors he used were reluctance motors, the power source being zinc-iron batteries. It was named Galvani and was demonstrated on the E \& GR that autumn, when it achieved a speed of 4 mph (6.4 km/h) while hauling a load of 6 tons over a distance of 1 1/2 miles (2.4 km); it was the first electric locomotive. Nevertheless, further support from the railway company was not forthcoming, although to some railway workers the locomotive seems to have appeared promising enough: they destroyed it in Luddite reaction. Davidson staged a further exhibition in London in 1843 without result and then, the cost of battery chemicals being high, ceased further experiments of this type. He survived long enough to see the electric railway become truly practicable in the 1880s.

[br]

PJGR / GW

SUBJECT AREA: Broadcasting, Electronics and information technology, Recording

[br]

[br]

[br]

Poniatoff was educated at the University of Kazan, the Imperial College in Moscow, and the Technische Hochschule in Karlsruhe, gaining degrees in mechanical and electrical engineering. He was in Germany when the First World War broke out, but he managed to escape back to Russia, where he served as an Air Force pilot with the Imperial Russian Navy. During the Russian Revolution he was a pilot with the White Russian Forces, and escaped into China in 1920; there he found work as an assistant engineer in the Shanghai Power Company. In 1927 he immigrated to the USA, becoming a US citizen in 1932. He obtained a post in the research and development department of the General Electric Company in Schenectady, New York, and later at Dalmo Victor, San Carlos, California. During the Second World War he was involved in the development of airborne radar for the US Navy.

In 1944, taking his initials to form the title, Poniatoff founded the AMPEX Corporation to manufacture components for the airborne radar developed at General Electric, but in 1946 he turned to the production of audio tape recorders developed from the German wartime Telefunken Magnetophon machine (the first tape recorder in the truest sense). In this he was supported by the entertainer Bing Crosby, who needed high-quality replay facilities for broadcasting purposes, and in 1947 he was able to offer a professional-quality product and the business prospered.

With the rapid post-war boom in television broadcasting in the USA, a need soon arose for a video recorder to provide "time-shifting" of live TV programmes between the different US time zones. Many companies therefore endeavoured to produce a video tape recorder (VTR) using the same single-track, fixed-head, longitudinal-scan system used for audio, but the very much higher bandwidth required involved an unacceptably high tape-speed. AMPEX attempted to solve the problem by using twelve parallel tracks and a machine was demonstrated in 1952, but it proved unsatisfactory.

The development team, which included Charles Ginsburg and Ray Dolby, then devised a four-head transverse-scan system in which a quadruplex head rotating at 14,400 rpm was made to scan across the width of a 2 in. (5 cm) tape with a tape-to-head speed of the order of 160 ft/sec (about 110 mph; 49 m/sec or 176 km/h) but with a longitudinal tape speed of only 15 in./sec (0.38 m/sec). In this way, acceptable picture quality was obtained with an acceptable tape consumption. Following a public demonstration on 14 April 1956, commercial produc-tion of studio-quality machines began to revolutionize the production and distribution of TV programmes, and the perfecting of time-base correctors which could stabilize the signal timing to a few nanoseconds made colour VTRs a practical proposition. However, AMPEX did not rest on its laurels and in the face of emerging competition from helical scan machines, where the tracks are laid diagonally on the tape, the company was able to demonstrate its own helical machine in 1957. Another development was the Videofile system, in which 250,000 pages of facsimile could be recorded on a single tape, offering a new means of archiving information. By 1986, quadruplex VTRs were obsolete, but Poniatoff's role in making television recording possible deserves a place in history.

Poniatoff was President of AMPEX Corporation until 1955 and then became Chairman of the Board, a position he held until 1970.

[br]

KF / GB-N

SUBJECT AREA: Civil engineering

[br]

[br]

[br]

In 1775 Pritchard designed the Ironbridge bridge, which was built over the River Severn by Abraham Darby of Coalbrookdale in 1777–9. It is constructed of five parallel arch ribs almost 200 ft (61 m) in length. The spandrels are filled by circles and ogee arch heads, while the roadway above is made from cast-iron plates 2½ in. (64 mm) thick. The bridge, which weighs 400 tons, was made in the Coalbrookdale foundry and took only three months to erect.

SUBJECT AREA: Canals, Civil engineering, Weapons and armour

[br]

[br]

[br]

William Twiss entered the Ordnance Department at the age of 15, and in 1762, aged 17, he was appointed Overseer of Works at Gibraltar. At the end of the Seven Years War, in 1763, he was commissioned Ensign in the Engineers, and further promotion followed while he still remained in Gibraltar. In 1771, as a Lieutenant, he returned to England to be employed on Port-smouth's dockyard fortifications. In 1776 he was posted to Canada, where he was soon appointed Controller of Works for the building of a British fleet for Lake Champlain. He was involved in military operations in the American War of Independence and in 1777 was present at the capture of Fort Ticonderoga (New York State). He was taken prisoner shortly afterwards, but was soon exchanged, and a year later he was promoted Captain.

In 1779 he was given the task of constructing a short canal at Coteau du Lac, Quebec, to bypass rough water at this point in the St Lawrence River between Montreal and Pointe Maligne. This was probably the first locked canal in North America. In 1781, following his appointment as Chief Engineer for all military works in Canada, he supervised further navigational improvements on the St Lawrence with canals at Les Cèdres and the Cascades. In parallel with these projects, he was responsible for an amazing variety of works in Canada, including hospitals, windmills, store-houses, barracks, fortifications, roads, bridges, prisons, ironworks and dams. He was also responsible for a temporary citadel in Quebec.

In 1783 he returned to England, and from 1794–1810 he served as Lieutenant- Governor of the Royal Military Academy at Woolwich, although in 1799 he was sent to Holland as Commanding Engineer to the Duke of York. In 1802 he was promoted Colonel and was in Ireland reporting on the defences there. He became Colonel Commandant, Royal Engineers, in 1809, and retired two years later. In retirement he was promoted Lieu tenant-General in 1812 and General in 1825.

[br]

SUBJECT AREA: Architecture and building

[br]

[br]

[br]

Born into a High Church and Royalist family, the young Wren early showed outstanding intellectual ability and at Oxford in 1654 was described as "that miracle of a youth". Educated at Westminster School, he went up to Oxford, where he graduated at the age of 19 and obtained his master's degree two years later. From this time onwards his interests were in science, primarily astronomy but also physics, engineering and meteorology. While still at college he developed theories about and experimentally solved some fifty varied problems. At the age of 25 Wren was appointed to the Chair of Astronomy at Gresham College in London, but he soon returned to Oxford as Savilian Professor of Astronomy there. At the same time he became one of the founder members of the Society of Experimental Philosophy at Oxford, which was awarded its Royal Charter soon after the Restoration of 1660; Wren, together with such men as Isaac Newton, Robert Hooke, John Evelyn and Robert Boyle, then found himself a member of the Royal Society.

Wren's architectural career began with the classical chapel that he built, at the request of his uncle, the Bishop of Ely, for Pembroke College, Cambridge (1663). From this time onwards, until he died at the age of 91, he was fully occupied with a wide and taxing variety of architectural problems which he faced in the execution of all the great building schemes of the day. His scientific background and inventive mind stood him in good stead in solving such difficulties with an often unusual approach and concept. Nowhere was this more apparent than in his rebuilding of fifty-one churches in the City of London after the Great Fire, in the construction of the new St Paul's Cathedral and in the grand layout of the Royal Hospital at Greenwich.

The first instance of Wren's approach to constructional problems was in his building of the Sheldonian Theatre in Oxford (1664–9). He based his design upon that of the Roman Theatre of Marcellus (13–11 BC), which he had studied from drawings in Serlio's book of architecture. Wren's reputation as an architect was greatly enhanced by his solution to the roofing problem here. The original theatre in Rome, like all Roman-theatres, was a circular building open to the sky; this would be unsuitable in the climate of Oxford and Wren wished to cover the English counterpart without using supporting columns, which would have obscured the view of the stage. He solved this difficulty mathematically, with the aid of his colleague Dr Wallis, the Professor of Geometry, by means of a timber-trussed roof supporting a painted ceiling which represented the open sky.

The City of London's churches were rebuilt over a period of nearly fifty years; the first to be completed and reopened was St Mary-at-Hill in 1676, and the last St Michael Cornhill in 1722, when Wren was 89. They had to be rebuilt upon the original medieval sites and they illustrate, perhaps more clearly than any other examples of Wren's work, the fertility of his imagination and his ability to solve the most intractable problems of site, limitation of space and variation in style and material. None of the churches is like any other. Of the varied sites, few are level or possess right-angled corners or parallel sides of equal length, and nearly all were hedged in by other, often larger, buildings. Nowhere is his versatility and inventiveness shown more clearly than in his designs for the steeples. There was no English precedent for a classical steeple, though he did draw upon the Dutch examples of the 1630s, because the London examples had been medieval, therefore Roman Catholic and Gothic, churches. Many of Wren's steeples are, therefore, Gothic steeples in classical dress, but many were of the greatest originality and delicate beauty: for example, St Mary-le-Bow in Cheapside; the "wedding cake" St Bride in Fleet Street; and the temple diminuendo concept of Christ Church in Newgate Street.

In St Paul's Cathedral Wren showed his ingenuity in adapting the incongruous Royal Warrant Design of 1675. Among his gradual and successful amendments were the intriguing upper lighting of his two-storey choir and the supporting of the lantern by a brick cone inserted between the inner and outer dome shells. The layout of the Royal Hospital at Greenwich illustrates Wren's qualities as an overall large-scale planner and designer. His terms of reference insisted upon the incorporation of the earlier existing Queen's House, erected by Inigo Jones, and of John Webb's King Charles II block. The Queen's House, in particular, created a difficult problem as its smaller size rendered it out of scale with the newer structures. Wren's solution was to make it the focal centre of a great vista between the main flanking larger buildings; this was a masterstroke.

[br]