modifying the length and thickness of the footnote line

[Rachael Thomas Carlson]

Hello all:

I am attempting to modify the thickness, placement, and length of the
spanner that appears at the bottom of the page when a footnote is
present in the code.  I have tried everything that comes to mind.

I kind of expect it to be 

\override FootNoteSpanner #'thickness = #'0.01

But that isn't it.  Does one have to modify the Footnote_engraver?

I guess that I am at a loss here.

Also this raises another question: what is the difference between
FootNoteItem and FootNoteSpanner?

Here is a snippet to work with:

\version "2.16.2"
{ 
c4\footnote #'(-1 . -1) \markup { blah blah blah } e g e 
}

Thank you very much for you consideration.

Rachael