First: I know how everyone reads cannonmn post, so I thought I'd slip this in before I answered his questions.
I want to thank everyone for their concern regarding my absense and subsequent condition. It really is gratifying to realize that so many of you actually read and possibly look foward to my drivel and inane mental wanderings in posts. As Mark Twain once submitted to a newspaper, "The reports of my death are greatly exaggerated".
What follows, are bits & pieces regarding the Jumel cannons.
16 Nov. 1904; D.A.R. Magazine.: "Exercises at Fort Washington attending the mounting of a cannon taken from the grounds of the Jumel Mansion and removed to Fort Washington. A salute was fired as the cannon was unveiled".
1905; St. Nicholas . A book by Mary Mapes Dodge.: "On the sloping grounds there are several old cannons".
May 14, 1916; New York Times.: "..........one of the original Revolutionary War cannon lies in the sod in the doorway".
1970s; "When there were cannons in front of the mansion".
I found a photograph somewhere that had a cannon on a wheeled carriage on the lawn, close to the steps, just to the right of the entrance as you view the building. It was so dark and blotched, it looked more like an ink-blot test.
If information cannot be gotten from the Arsenal, try the Jumel Chapter of the D.A.R. and the chapter of Colonial Dames. They share the Mansion. The Museum of the City of New York may have come into the picture. Then there is The New York Historical Society and the New York Public Library. Those are just off the top of my head.
If I were you, I would peruse the eBay listings for Post Cards and enter Jumel Mansion. That would be the easiest and probably most gratifying approach.
rc