<P> In the mid-5th century BC, when the Athenian Acropolis became the seat of the Delian League and Athens was the greatest cultural centre of its time, Pericles initiated an ambitious building project that lasted the entire second half of the century . The most important buildings visible on the Acropolis today--the Parthenon, the Propylaia, the Erechtheion and the temple of Athena Nike--were erected during this period . The Parthenon was built under the general supervision of the artist Phidias, who also had charge of the sculptural decoration . The architects Ictinos and Callicrates began their work in 447 BC, and the building was substantially completed by 432, but work on the decorations continued until at least 431 . </P> <P> The Parthenon is a peripteral octastyle Doric temple with Ionic architectural features . It stands on a platform or stylobate of three steps . In common with other Greek temples, it is of post and lintel construction and is surrounded by columns ("peripteral") carrying an entablature . There are eight columns at either end ("octastyle") and seventeen on the sides . There is a double row of columns at either end . The colonnade surrounds an inner masonry structure, the cella, which is divided into two compartments . At either end of the building the gable is finished with a triangular pediment originally occupied by sculpted figures . The columns are of the Doric order, with simple capitals, fluted shafts and no bases . Above the architrave of the entablature is a frieze of carved pictorial panels (metopes), separated by formal architectural triglyphs, typical of the Doric order . Around the cella and across the lintels of the inner columns runs a continuous sculptured frieze in low relief . This element of the architecture is Ionic in style rather than Doric . </P> <P> Measured at the stylobate, the dimensions of the base of the Parthenon are 69.5 by 30.9 metres (228 by 101 ft). The cella was 29.8 metres long by 19.2 metres wide (97.8 × 63.0 ft). On the exterior, the Doric columns measure 1.9 metres (6.2 ft) in diameter and are 10.4 metres (34 ft) high . The corner columns are slightly larger in diameter . The Parthenon had 46 outer columns and 23 inner columns in total, each column containing 20 flutes . (A flute is the concave shaft carved into the column form .) The roof was covered with large overlapping marble tiles known as imbrices and tegulae . </P> <P> The Parthenon is regarded as the finest example of Greek architecture . The temple, wrote John Julius Cooper, "enjoys the reputation of being the most perfect Doric temple ever built . Even in antiquity, its architectural refinements were legendary, especially the subtle correspondence between the curvature of the stylobate, the taper of the naos walls and the entasis of the columns ." Entasis refers to the slight swelling, of 1 / 8 inch, in the centre of the columns to counteract the appearance of columns having a waist, as the swelling makes them look straight from a distance . The stylobate is the platform on which the columns stand . As in many other classical Greek temples, it has a slight parabolic upward curvature intended to shed rainwater and reinforce the building against earthquakes . The columns might therefore be supposed to lean outwards, but they actually lean slightly inwards so that if they carried on, they would meet almost exactly a mile above the centre of the Parthenon; since they are all the same height, the curvature of the outer stylobate edge is transmitted to the architrave and roof above: "All follow the rule of being built to delicate curves", Gorham Stevens observed when pointing out that, in addition, the west front was built at a slightly higher level than that of the east front . </P>

What was the roof of the parthenon made of