Complex sentence: At Boulogne he was arrested and condemned to twenty years' imprisonment in a fortress, shortly afterwards commuted into mild detention at Versailles, where he wrote a book to prove that the Pyramids were built to prevent the Nile from silting up.
Split into simpler sentences: At Boulogne he was arrested and condemned to twenty years' imprisonment in a fortress, shortly afterwards commuted to mild detention at Versailles. There he wrote a book to prove that the Pyramids were built to prevent the Nile from silting up.