To add a little more... Just looked at my copy of Seacoast Fortifications of the United States An Introductory History by Emanuel Raymond Lewis (1979). It shows a pair of (replica) Model 1841 Seacoast guns at Ft. Washington, location looks identical to the photograph.
These gun positions would have been for smaller guns than the 15 inch Rodmans, but most likely similar smoothbore guns. If you want to see actual 15 inch Rodmans in position where they were originally installed, just up the road from Fort Washington is Fort Foote, where a pair of the Rodman cannon sit on their carriages in the woods.