Extra Extra Porter

Remember my promise yesterday? Which promise? ("I promise lots of things. That's why I'm such a good father.") The one to work out the ABV for the Porter and Brown Stout OGs and FGs listed in the 1824 Encyclopaedia Britannica.

I'm a man of my word, so here they are:

Brown Stout: OG 1062.4, FG 1017, 5.9% ABV

Porter: OG 1053.5, FG 1013, 5.3% ABV