You would not likely get in trouble using either wherever you
might want one or the other. Many people, however, like to
make this distinction: "farther" when there is a distance,
either a real distance or the clear concept of distance,
involved, and "further" when the sense is largely removed
from an actual stretch.
Three miles farther on we came to an inn.
As his thoughts wandered farther from her being gone,
his depression faded.
We can do nothing further without better information.
Zeppo: Will there be anything further, father?
Groucho: Surely that should be anything father further?
--
Cordially,
Eric Walker, webmaster
Great Science-Fiction & Fantasy Works
http://owlcroft.com/sfandf
--Odysseus
'farther' is a comparative of 'far' meaning 'more distant' as in: 'There's
another shop farther down the road.'