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, 1922).

The book has been scanned and can be read <A HREF="http://imgbase-scd-ulp.u-strasbg.fr/displayimage.php?album=433&pos=0">here</A>.