Usage Discussion of FARTHER
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
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 farther shore> and further the meaning of addition further invitation>.