TV's Frink said:
usage Farther and further have been used more or less interchangeably throughout most of their history, but currently they are showing signs of diverging.
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.
Yes; this is true, but I am determined to resist attempts to polarise time-honoured variant spellings.
The rest is all double Dutch to me. :-)