Well, I found out where the barometric reading was taken, at what’s now known as Craig Key, a small island constructed for the railroad. The reading was either taken out in the open or in a dry docked boat, according to two different accounts.
https://en.wikipedia.org/wiki/Craig_Key
And at the following link the measurement is described in item 7.
http://www.keyshistory.org/35-hurr-war-dept.html
Interesting, thanks