Farther and further have been used more or less interchangeably throughout most of their history, but currently they are showing signs of diverging. As adverbs they continue to be used interchangeably whenever spatial, temporal, or metaphorical distance is involved. But where there is no notion of distance, further is used <our techniques can be further refined>. Further is also used as a sentence modifier <further, the workshop participants were scarcely optimistic — L. B. Mayhew>, but farther is not. A polarizing process appears to be taking place in their adjective use. Farther is taking over the meaning of distance <the farther shore> and further the meaning of addition <needed no further invitation>.
Examples of FARTHER
It's farther away than I'd thought.
<they had traveled farther down the Colorado River than any previous explorers>