Poncelet published his theorem ("Poncelet's porism) in 1822, after he returned to France following his captivity as war prisoner in Russia: 

J.V. Poncelet, Traité des propriétés projectives des figures (Paris, 1822).

The book has been scanned and can be read [here][1] or on [archive.org][2].


  [1]: https://docnum.unistra.fr/digital/collection/coll7/id/71696
  [2]: https://archive.org/details/traitdesproprit00goog/page/n7/mode/2up