Final answer:
The central idea of the excerpt is the disbelief and subsequent admiration for the Brooklyn Bridge's ambitious and successful construction, representing a milestone in technological advancement and engineering.
Step-by-step explanation:
The central idea of the excerpt from the opening ceremonies speech given by Hon Seth Low, the former mayor of Brooklyn, is about the incredulity and marvel surrounding the conception and eventual realization of the Brooklyn Bridge. When the bridge was first proposed, the idea of a massive suspension bridge that could connect Brooklyn and Manhattan was met with skepticism. However, over time the bridge was completed and stood as a testament to technological progress and engineering prowess, symbolizing the potential of human ingenuity to overcome natural barriers and connect communities.