Sorry, but I am going to have to amend this one. Hmea1's prescription is no longer a valid one, though it was a wish of some grammarians years ago. That dictum (starting with Vizetelly in 1906) did not account for the educated usage of his own time or for that of the past. 'Farther' and 'further' are historically the same word, and they have long been used more or less interchangeably.
That is the truth, Ali. However, if you submit a written piece to a teacher or a publisher, they may have their own opinions which you will have to accept.