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.
The observatory built in the 1820s was the best place to see the stars.
mnli hypothesis: The observatory built in the 1820s was the best place to see the stars. 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.