From this site:

Farther or Further?

Farther refers to length or distance. It is the comparative form of the word far when referring to distance.

Further means "to a greater degree," "additional," or "additionally." It refers to time or amount. It is the comparative form of the word far when meaning "much."

    Correct: London is farther north than Juneau.
    (Refers to distance)

    Correct: This plan requires further study.
    (Meaning "additional study," refers to amount)

    Correct: According to my timetable, we should be further along.
    (Refers to time)