FROM KIMMYPIE
How do you know when to use further or farther?
RESPONSE
The most common rule is that “further” generally applies to degree (as in more of something) and “farther” generally applies to physical distance, like this:
The issue requires further debate.
Harbertown is farther from us than Ellington.
Often, you'll find refinements to the rule that permit “further” in places that normally might require “farther”. Others say the two words are interchangeable in certain circumstances.
I could never remember all the refinements of those rules, and they always struck me as if some mincing spinster just made them up one day. They also never satisfied my writing intuition, so I invented my own rules. I use “further” when speaking of the progression of one thing and “farther” when speaking of a comparison between two or more things. If you abide by this rule, then “further” is pretty much a synonym for “more” or “additional”, and there is no such word as “furthest”.
FURTHER
After the initial bite, the fangs sank further into the soft flesh.
We endured the formative part of his argument, and then he expounded further.
We've traveled 200 miles; shall we travel further?
FARTHER
The fangs sank farther into the soft flesh than the claws.
While both reports conceded his eccentricities, the second went farther and dubbed him insane.
People traveled from all over to attend the party, but Bill traveled the farthest.
Of course, I know many will say my rule is completely wrong, to which I say, “Fine. Memorize the traditional rules and good luck to you. My rule doesn't hurt meaning as far as I can tell, and it's a heckuva lot easier to remember.”