Sir

It is nice to see fourteenth-century Bolognese frescoes getting a bit of well-deserved publicity in the Science in Culture article “St Christopher and the vortex”1. However, if Theodore von Kármán was inspired by the beautiful San Domenico fresco of St Christopher to conceive of his ‘vortex street’, his powers as a psychic would be at least as remarkable as his achievements in the physical sciences.

True, the fresco in the church of San Domenico was painted during the late fourteenth century (to a formula much used by Tomaso da Modena and his associates2) and Kármán was born in 1881. But it had been hidden since 1728–31 behind a staircase built up to an organ loft. It was only revealed during repairs in 1979.

So what was the painting that Kármán reported had inspired him? Mizota and his partners are surely correct in seeing fourteenth-century painting style in Kármán's water-flow representation, but today remarkably few examples of St Christopher survive in Bologna itself: the grandest, in San Petronio, by Giovanni da Modena (circa 1410–20), has been cut off at the ankles. Tomaso da Modena painted a similar image in San Francesco, Treviso, around 1350. A pupil of his painted another in the next chapel; similar accumulations probably occurred in churches in Bologna, but were lost during the Counter-Reformation.

The representation of St Christopher that Kármán is most likely to have seen is the triptych by Jacopo di Paolo from the 1390s that has long been shown in various rooms of the outer cloisters of San Stefano3 — but this comes nowhere near the San Domenico fresco in the clarity with which it shows water flowing.

We might deduce from this that von Kármán was sufficiently sensitive to painting style to have read the appropriate patterns into what he saw, from his own knowledge of both art and water, in his moment of inspiration.