Run out from Post

This is one of several that comes with a little quote:

You are my aspiration, I own few blogs and often run out from post :). "'Tis the most tender part of love, each other to forgive." by John Sheffield.

I am sure Sheffield would be so proud that is quote was used in such a valuable manner. Assuming this is
John Sheffield, 1st Duke of Buckingham and Normanby, and not John Sheffield, the actor who played Tarzan.