The largest amount of information I needed for my research could be find in the third Piranesi’s drawing. It represents, on two levels, the rear elevation of the temple (including the ekklesiasterion on the back) and a lateral transection of the whole iseum.
However, when I applied the scaling operations on this drawing, I was very disappointed to see that it didn’t match the other two (the plan and the front elevation) . I double checked the process but there were no mistakes in the maths.
Although I could have just manually rescaled the mismatching drawing in order to make it fit with the rest of my model, I wanted to pursue a more rigorous approach and verify if the measurements I have derived (through geometric and algebraic operations) from the first two sources were reliable.
Consequently, I decided to cross reference the dimensions derived from Piranesi’s plan against the data recorded by John Soane.
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| Detail of Soane's plan fro the Temple of Isis at Pompeii. Courtesy of the Soane Museum |
As I had already noticed, the shape of the two plans (Piranesi’s and Soane’s) look quite consistent so I expected Soane’s measurements to confirm the ones I had already derived from Piranesi’s previous drawings.
On the contrary, the numbers cited by Soane appear to be hugely different from the ones I derived from Piranesi. According to Soane, the length of the iseum's colonnade is 46’9’’ english foot. It means circa 14,23 meters, while according to my calculation from Piranesi, the length should be 19,363 meters (x 22,795 meters).
What I was supposed to do with two sources apparently very consistent but actually so different? I wanted to identify which of the two was the most reliable. The only way to verify this information was to look for hard measurements of the iseum. Eventually, I found them cited in the book "Alla ricerca di Iside", by Stefano De Caro, a very well known Pompeian scholar. The dimensions recorded by De Caro are a very encouraging match with the ones derived by Piranesi (19,78 x 22,7 meters). Consequently, I discarded Soane’s plan and focused on Piranesi’s first two drawings. However, I am still curious to discover how is possible that such an expert and passionate architect recorded wrong measurements.
Going back to my first problem (the little discrepancy between the first two Piranesi’s drawings and the third one), in order to complete my dissertation project I felt it was sensible to slightly rescale the third Piranesi’s drawing to make it match with the previous two that have been confirmed by modern hard measurements.
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