www.lesswrong.com/posts/Djcwo4GTfLTEDuvx4/did-anyone-predict-the-industrial-revo...

Huygens's own 22 September 1673 letter says he had already shown an experimental version of the invention to members of the Académie and to Colbert: "J'ay fait voir ... un essay d'une invention" ("I have shown ... an experiment/trial of an invention"). In the same letter he gives dimensions for test cylinders he had used and adds that, with a tube one foot in diameter, he had shown "des effects surprenans a elever des poids et des hommes"—surprising effects in lifting weights and men. That is direct contemporary evidence that some form of the gunpowder engine was in fact constructed and demonstrated.

A modern history article from the Royal Society of Chemistry likewise describes Huygens as demonstrating a prototype of the gunpowder engine, saying it could lift "seven or eight boys" with a small amount of gunpowder. So the defensible claim is that Huygens's engine was not developed into a practical large-scale machine, or that historians debate its feasibility at scale. But saying it "was never actually constructed" is contradicted by both Huygens's own report and later historical summaries.