function ShowFooter(){
  var zFooter = document.getElementById('FooterDiv');
  var zNav = document.getElementById('LeftNavigation');
  var zContent = document.getElementById('MainContent');
  var zHeight = (zNav.clientHeight>zContent.clientHeight ? zNav.clientHeight:zContent.clientHeight);
  zFooter.style.top = zHeight+'px';
  zFooter.style.display = 'block';
}