Then or Thence?

Hello, I am reading a book called "To Kill a Mockingbird" and at beginning there's a sentence which made me wonder about a word's meaning. Honestly, I don't know what it means. I've never seen this before, but I searched on Google about it and people say that It means the same of "then". Can tell me what this word means and what's its difference from "then"?

"He worked his way cross the Atlantic to Philadelphia, thence to Jamaica, thence to mobile, and up the Saint Stephens."


Thanks in advance.