This dictionary was first published in 1848 and while it is a valuable resource, it does not do exactly what it purports to. Many of these words were known to be used in England, either at the time of it being written, or as obsolete provincialisms. Words like that are not exactly what I would call "Americanisms". I definitely felt I had to double-check the etymologies against other resources to determine if they were truly American inventions or not.
That aside, it was interesting to see just how big of an impact America had on the language, donating some absolute bangers such as: cahoots, bender, rowdy, scrumptious, snoop, spooky, skedaddle, hunky-dory, and bee-line, to name just a few.