Jane Austen by Cassandra Austen © National Portrait Gallery, London
Primary sources are original, uninterpreted information, such as firsthand accounts of events in letters, diaries, interviews, or historical news reportage.
Searching the text of documents from late 18th-early 19th century can be a bit tricky.
Modern terms weren't necessarily used! Keep track of any unusual phrases you find, and use them to search.
Spelling was not yet standardized, so be flexible and include alternate spellings!
Try using OR to combine several spellings of search words or synonyms, such as:
smallpox OR smalpox OR small-pox
Also try truncation to get variations in spelling that have the same root:
adverti* to get advertize OR advertise