How did James Fergusson arrive at his conclusion about Sanchi's association with serpent worship?

By analyzing images without knowledge of Buddhist literature

The motif of the serpent, present on numerous pillars at Sanchi, warrants our consideration. The emblem appears to draw from prevailing traditions that weren't necessarily documented in texts. Intriguingly, even one of the earliest modern art historians, James Fergusson, perceived Sanchi as a hub of tree and serpent veneration. It's noteworthy that Fergusson's understanding was shaped without access to a comprehensive understanding of Buddhist literature, most of which remained untranslated during his time. His conclusion stemmed solely from studying the visual representations in isolation.