function [ dH ] = d_bottom_H(x)
% 
% Written by:
% -- 
% John L. Weatherwax                2005-06-15
% 
% email: wax@alum.mit.edu
% 
% Please send comments and especially bug reports to the
% above email address.
% 
%-----

if( abs(x) <= 40 )
  dH = -(x/80);
else
  dH = 0;
end