farther - correct spelling
farther adverb and adjective
Note: Some usage guides insist that only farther describes physical distance (We walked farther than we planned). But farther and further have been used interchangeably throughout much of their histories.
Grammar.com’s section on Problem Words discusses fartherand further. Click here for that discussion.
Example: We walked farther than we planned. adverb
Example: We visited the farther side of the mountain. adjective