When the author relates a particular event, he encourages the reader to follow along. When this happens, the reader is meant to focus on the story and understand the events that transpired. However, the reader might get distracted, or he might fail to identify the specific aspects that the author wants to highlight.
By using contrast and contradictions, the author encourages the reader to focus on a new idea that is significantly different to the one already presented. By doing so, the author forces the reader to pause, rethink and reevaluate the information presented, thus increasing his understanding of the subject.
using a list of SOURCES is the best way to speed up the research on the project
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.