The Pantheon

I had to study this for Art History 1, and thought it would be a good Form & Space post:
The Pantheon is the best-preserved example of monumental Roman architecture. It consists of a large top atop a rotunda. The thickness of the dome varies from 21 ft at the base of the dome to 4 ft around the oculus. The 5,000 tn weight of the concrete dome is concentrated on a ring of voussoirs 30 ft. in diameter which form the oculus while the downward thrust of the dome is carried by eight barrel vaults in the 21 ft. thick drum wall into eight piers. It was built of a very strong concrete with pozzolona cement. A gradation process was used so that the structure is heavier at the bottom and much lighter at the top. The dome's oculus or opening lightens the load and acts as a compression ring. The dome is a masterpiece of engineering: it is the largest dome ever covered by masonry and it was cast in a single operation on an imposing wooden centering. The Pantheon still holds the record for the largest unreinforced concrete dome in the history of architecture.