We often see a similar accent used in pop culture and media for characters in early America. How can we know or why do we assume this is the way people spoke in Revolutionary America? Furthermore, when did the accent begin to sound like the American accent we hear now-a-days?
I know this one! I know this one!
Basically, historians reconstruct accents through writing. One method is by looking at poetry. There are lots of words that used to rhyme and don't anymore. Example: there's this poem by William Blake called 'Tiger, Tiger'. Part of it rhymes 'eye' with 'symmetry'. We can assume that one or both words were pronounced slightly differently. Another method is by looking at documents to find misspellings. When historical figures frequently spell a word differently, and they're all spelling it in a particular sort of way, it's easy to figure that the vowels are different.