People use both further and farther to mean “more distant.” However, American English speakers favor farther for physical distances and further for figurative distances.
Ray LeBlond once said “You learn something every day if you pay attention.” Today is the day to learn the difference between further and farther.
Farther
Unsurprisingly, farther means “at or to a greater distance.” In Salt to the Sea, Ruta Sepetys uses this adverb to describe the activity of some sea vessels: Some boats eventually floated ashore.