This is a much misunderstood comparison, since both words can be used in an interchangeable manner. Fashion and usage have brought ‘further’ into greater use for most things. The use of the term ‘further education’ has become so well-known that no one would ever think of writing this as ‘farther education’, though such an expression wouldn’t be incorrect.
There is some debate about whether ‘farther’ should be preferred in those expressions where a measurable distance is concerned. But there is no etymological reason for this distinction and it has now been superseded by usage. It seems likely that ‘farther’ will be one of those words that will eventually vanish due to lack of use.
NOTE from the Shorter Oxford English Dictionary: The forms farther/further displaced farrer, the regular compar. of far. Until recently, farther was preferred in reference to physical distance, further in figurative contexts, but further is now usual in all contexts.
Once again, Zemanta has let me down, so here's another local shot; this is the Humber Bridge, until recently, the longest single span suspension bridge in the world.