An additional observatory was added in the 1820s, but the smoky skies and train steam over the city at that time made it an unsuitable location for watching the stars.
Train steam was the main reason that the observatory wasn't very good.
mnli hypothesis: Train steam was the main reason that the observatory wasn't very good. premise: An additional observatory was added in the 1820s, but the smoky skies and train steam over the city at that time made it an unsuitable location for watching the stars.