Used to separate to independent clauses when second explains illustrates the first
To begin with letter 3, It could be said that the main theme is determination because Walton feels confident about the voyage and he is ´determined' to go on with it despite the weather conditions.
As regards letter 4, we might say that the theme may be trust and friendship because Frankenstein and Walton met and he is will to tell him the his story. So this letter might be seen as the beginning of their friendship.
I would feel proud, becasue it tells what she had to conquer as a woman in 1650. It was a very hard time for women back then so being described as a respectable person would be very rare for women.
