Here’s the wood bridge across the dry moat at Fort Jay (on Governors Island) in 1891. Given that horses and mules were the army’s transport at that time other than soldiers walking, I assume the elevated sidewalks were to allow people to stay out of the animals’ droppings.
The wood deck was replaced in the 1950s with a concrete deck supported by the old stone piers. Marie headed up an OSE project for the National Park Service in designing repairs to the concrete deck and in studying the feasibility of putting back a timber deck.
The fort walls were completed in its current layout in 1809; and the buildings were completed in their current configuration in the 1830s. There have been many small renovations and alterations since: this structure is over 200 years old and almost nothing survives that long unaltered.
Given the high level of masonry skill needed to build the walls, it was certainly possible for the Army Corps of Engineers to have built a stone-arch bridge across the moat. (Keeping in mind that in the US in 1809, a timber bridge and a stone-arch bridge were literally the only options.) And the cost of a stone bridge would have been small compared with the cost of the fort. The use of a wood deck made sense when I stopped thinking like a civil engineer and tried thinking like a military engineer.
Unlike Castle Williams, not far away, Fort Jay is not right up against the shore of the island. So the moat would only ever be used for its intended purpose – making it difficult for infantry to attack the fort at close range – if enemy troops were on the island in large numbers. In that scenario, the bridge would be a liability, allowing the enemy to get close to the entrance doors, which are the weakest point in the perimeter walls. Having a readily-destroyed bridge is an advantage in that case. Of course, in literally any other circumstances, having a stronger bridge that required less maintenance would be good.
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