You learn something everyday...
It's been a few years since I've been graded on my grammar skills so when putting together my 'Grateful' post of the day I wasn't sure which was correct, 'farther' or 'further'.
In case you were wondering (which you probably weren't)
Farther shows a relation to physical distance. If you can replace the word farther with "more miles" then you have done it correctly.
Further relates to metaphorical distance or depth. It is a time, degree, or quantity. It is also another way of saying "additional".
How 'bout that?