Searching full-text primary documents from the 18th century can be a problem, either because of mistakes transforming the text to something machine-readable, or because 18th century spelling was not yet standarized.
To combat this, try combining several spellings of search words, 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
Work in progress...I'll be adding to this section as the semester proceeds.