Thersilion (thing)

Bouleuterion in ancient Greece, which used an innovative approach to the Greeks' persistent trouble with roof supports.

The Problem

Even with voting limited to the free male landowners, an ancient Greek city-state could have an electorate numbering in the thousands. Civic meetings could be held in the local open air theater during the summer, but in mountainous regions, citizens want to be inside in the winter.

The problem was made worse because of the relatively unsophisticated building techniques the Ancient Greeks used. They had not discovered truss roofing, so all of their structures used post and lintel techniques to hold the roofs up. But post and lintel construction requires the columns to be very closely spaced: the maximum intercolumnial distance they could achieve was about 22 feet (6.7 meters). As a result, large roofed spaces tended to become clogged with forests of columns.¹

The Standard Solution

The standard arrangement of columns in a roofed space was a grid. A good example of this was the Hall of the Mysteries in Eleusis, built in the fifth century BCE. This building perfectly illustrates the main problem arising from gridded columns: visibility. As the diagram below shows, there are only four lines of view to a speaker standing at point x. Anyone not in one of those lines would be blocked by one or more columns.

 _____________________________
|===========seating===========|
||===========================||
|||o   o   o 1 o   o   o   o|||
|||          |              |||
|||o   o   o | o   o   o   o|||
|            V                | <- windows
|||o   o   o   o   o   o   o|||
|||2-------> x <-----------3|||
|||o   o   o ^ o   o   o   o|||
|||          |              |||
|||o   o   o | o   o   o   o|||
|            |                | <- windows
|||o   o   o 4 o   o   o   o|||
||======   =========   ======||
|=======   =========   =======|
o                             o
o  o o  o  o  o o  o  o  o o  o

 N        0   20  40  60  80 feet
/         |---|---|---|---|

A Novel Approach

In the fourth century BCE, the citizens of Megalopolis in Arcadia built (or rebuilt; the records are not clear) a bouleuterion called the Thersilion. The building was 172 x 218 feet (52.5 x 66.5 m) and was designed to hold all 10,000 voting citizens.

The Thersilion was destroyed along with the rest of Megalopolis by the Spartan Cleomenes III in 222 BCE, and only the floor remains. No contemporaneous description of the inside of the building survives, although it was apparently famous for its front porch. It was excavated in 1890 by the American School in Athens, and has not been investigated since. Because of the state of the building's remains, and because of the scarce archaeological attention it has recieved, the Thersilion's features will be divided into "facts" and "theories".

Fact: Column Placement

In accordance with the Athenian fashion for bouleuteria, citizens were seated in a squared-off U shape around a central speaker. The columns to hold the roof were also set in concentric squared-U shapes, again very common in Greek bouleuteria at the time. However, the traditional bouleuterion had its columns evenly spaced in a grid, leading to the problems illustrated by the Hall of the Mysteries above. In planning the Thersilion, someone had a stroke of genius. By varying the spacing in the U's of columns, a radial pattern could be created. This would improve the lines of sight and allow light from the windows into the central speaker's area.

The final layout looked something like this. Note that people in all 16 areas can see a speaker standing at the x (I have not drawn the lines of sight. The diagram is confusing enough as it is.).

            oo o o o o o o o o o o oo <- NB: columns were evenly spaced
            o                       o
 ___________|   o   o       o   o   |______________
|                                                  |
|  1                                           2   |
|        o                               o         |
|            o   o   o       o   o   o             |
|    o                                       o     |
|  3     o           o       o           o     4   |
|            o                       o             |
|                o               o                 |
|    o   o   *           x           *   o   o     |
|                o               o                 |
|  5         o       o       o       o         6   |
|        o                               o         |
|    o                                       o     |
|            *   o    o     o    o   *             |
|                                                  |
|  7     o                               o     8   |
|            o   *   o   *   o   *   o             |
|    o                                       o     |
|                                                  |
|  9     o     o    o    o    o    o     o    10   |
|                                                  |
|    o       o     o     o     o     o       o     |
|       11     12     13    14    15     16        |
|__________________________________________________|

 N           0     20    40    60    80 feet
  \          |-----|-----|-----|-----|

* These columns were added to strengthen the building, when the wide intercolumnial spans of the original design proved too weak.

The columns were, as stated above, set up in concentric squared-off U shapes. Below is a diagram showing the architraves that connected them, highlighting the U shapes. Each U is labelled with a letter, for ease of discussion.

     A   B   C   D   E       E   D   C   B   A

 ____________|---o---o-------o---o---|_____________
|    |   |   |   |   |       |   |   |   |   |     |
|    |   |   |   |   |       |   |   |   |   |     |
|    |   o   |   |   |       |   |   |   o   |     |
|    |   |   o---o---o-------o---o---o   |   |     |
|    o   |   |   |   |       |   |   |   |   o     |
|    |   o   |   |   o-------o   |   |   o   |     |
|    |   |   o   |   |       |   |   o   |   |     |
|    |   |   |   o   |       |   o   |   |   |     |
|    o   o   *   |   |   x   |   |   *   o   o     |
|    |   |   |   o   |       |   o   |   |   |     |
|    |   |   o   |   o-------o   |   o   |   |     |
|    |   o   |   |               |   |   o   |     |
|    o   |   |   |               |   |   |   o     |
|    |   |   *   o----o-----o----o   *   |   |     |
|    |   |   |                       |   |   |     |
|    |   o   |                       |   o   |     |
|    |   |   o---*---o---*---o---*---o   |   |     |
|    o   |                               |   o     |
|    |   |                               |   |     |
|    |   o-----o----o----o----o----o-----o   |     |
|    |                                       |     |
|    o-------o-----o-----o-----o-----o-------o     |
|                                                  |
|__________________________________________________|

Looked at that way, it becomes clear why the columns marked * were added. Without them, the spans in U C are the longest in the building. As will become clearer later on, it is probable that C bore the most weight as well.

Fact: Floor Height

The concentric U shapes were reflected in the floor, which was stepped like a theater. (The heights in the diagram have been exaggerated)

|    A   B   C   D   E       E   D   C   B   A    |
|______                                     ______|
|xxxxxx|___                             ___|xxxxxx|
|xxxxxxxxxx|___                     ___|xxxxxxxxxx|
|xxxxxxxxxxxxxx|___             ___|xxxxxxxxxxxxxx|
|xxxxxxxxxxxxxxxxxx|     x     |xxxxxxxxxxxxxxxxxx|

Theory: Lighting

It is almost certain that the walls of the Thersilion had windows in them, aligned with the radial lines of view. The excavators speculated that there was also an opaion, or clerestory over the central area where the speaker stood. This would have provided natural light for the central area, which would have been rather dark despite the windows. The best row to support the columns was probably C, even though it was originally the weakest of the five. The strain from supporting the clerestory would also explain the need to add more columns after the structure was built.

Theory: Roof Shape

The front of the building almost certainly had a peaked roof, to highlight the dramatic front porch. However, the back of the building was not nearly so dramatic, and would probably have had a hipped roof. The top view (with the clerestory removed for clarity) would therefore be something like this:

      Dramatic front porch
               |
               v
   _________________________
  |            |            |
  |            |            |
  |        |       |        |
  |        |       |        |
  |        |       |        |
  |            |            |
  |           / \           |
  |        /       \        |
  |     /             \     |
  |  /                   \  |
  |/                       \|

Conclusion

The Thersilion was a brilliant example of the refinement of existing technologies. Its creators finessed their contemporary architectural design to solve the main challenge of the bouleuterion form. Had it been designed in Athens in the fifth century BCE rather than in Megalopolis in the fourth, it would have been famous, and become the model for all subsequent bouleuteria throughout the Hellenic world.

Unfortunately, it was built in the dying days of Greek democracy, when tyrants and kings were less and less interested in the opinions of the citizens. And the Romans, who might have taken on the bouleuterion architectural form, had better, more efficient ways to roof over large areas for their citizens to vote in.

So the Thersilion at Megalopolis is virtually unknown, a landmark that might have been.

Footnote

1. Arches and domes, as the Romans demonstrated in spaces like the Pantheon, would have solved the problem. But although the Greeks were aware of the arch, they did not feel it was appropriate for most buildings. They only used arches in spaces that were either literally or conceptually underground, such as the athlete’s tunnel at the Nemean amphitheatre, and temples to Hades.