The Pantheon - The work of genius that stood the test of time

Few ancient buildings have survived the ravages of time so well as the Pantheon in Rome. It is the only example of similar age, size and span that remains intact. The fact is no accident - it's largely the result of superior engineering.

Often copied, sometimes equaled but never surpassed, it had more architectural innovations than most modern buildings. All the more remarkable, given that it was built around 125 AD under the aegis of the emperor Hadrian.

The plan is simple enough: a circular enclosure aside a rectangular entrance. The entrance sports a classic Greek portico of granite columns topped by a triangular pediment. There are three ranks of the 39 ft Corinthian supports, eight in front and two sets of four further in leading to the main rotunda. A rectangular section joins the portico to the rotunda.

But within that simple design are a dozen signs of genius.

The giant concrete dome topping the cylinder forming the major component was so well designed and built that no similar type would stand up under its own weight.

The dome is 142 feet in diameter (46 feet larger than that crowning the White House in Washington, DC), while the oculus at the peak is over 25 feet of that total. It stands as a result of its unusual composition, outstanding engineering and brilliant construction.

Just one example is the oculus in the center - the opening through the top. It decreases the overall weight and serves as a ring that distributes stress around its circumference. Imagine, by analogy, how difficult it is to crush a bicycle wheel.

It also serves to admit light to the interior. And rain, too it must be said, though the floor is an early example of slanting the floor toward drains.

The dome's tapering steps provide yet more evidence of the mastery of craft displayed by the dome's designer. It's 20 feet thick at the base, 7.5 at the oculus and composed of heavier material at the bottom, lighter as it rises. That doesn't seem so remarkable until one considers that many architects a thousand years later ignored this simple idea.

Nearly two thousand years after its birth the Pantheon in Rome is as stable today as when it was first built. Yet it was constructed without the benefit of machines or modern tools.

Nor did the Pantheon engineers have the advantage of modern transportation methods. All the materials were floated down the Tiber and moved to the site by man and animal on carts of the period.

Though its enormous bronze doors have been restored many times, no major structural work has ever had to be undertaken. This is all the more remarkable given the marshy land on which the structure is built.

By contrast, observe there have been several decades-long projects to preserve the Leaning Tower of Pisa, owing in part to the soft ground in parts of the site. The Parthenon in Greece, though a great building, was a virtual ruin 2,000 years after its birth.

The Pantheon in Rome was first converted to a church during the 8th century and continues to serve that purpose today. In fact, the building has been in continuous use since first being built.

This amazing building has often been copied, two notable examples being the British Museum Reading Room and the Thomas Jefferson Rotunda at the University of Virginia.

When in Rome, be sure to spend time viewing the original.