function getWindowHeight() {
    var windowHeight=0;
    if (typeof(window.innerHeight)=='number') {
        windowHeight=window.innerHeight;
    }
    else {
     if (document.documentElement&&
       document.documentElement.clientHeight) {
         windowHeight = document.documentElement.clientHeight;
    }
    else {
     if (document.body&&document.body.clientHeight) {
         windowHeight=document.body.clientHeight;
      }
     }
    }
    return windowHeight;
}
function setBackButton() {
var hauteur = 50;
//  alert("var hauteur = 300;");
  if (document.getElementById) {
  //alert("document.getElementById");
     var windowHeight = getWindowHeight();
     if (windowHeight > 0) {
    //        alert("windowHeight > 0");
         var footerElement = document.getElementById('back');
         if (windowHeight - hauteur >= 0) {
          if ((document.all) && (navigator.appVersion.indexOf("Mac",0)<0)) {
                footerElement.style.position = 'absolute';
                footerElement.style.top = (document.body.scrollTop + windowHeight - hauteur) + 'px';
                }
          else {
                footerElement.style.position = 'fixed';
                footerElement.style.top = (windowHeight - hauteur) + 'px';
                }
         }
         else {
            footerElement.style.position = 'static';
         }
     }
  }
}
window.onload = function() {
        setBackButton();
}
window.onresize = function() {
        setBackButton();
}
window.onscroll = function() {
        setBackButton();
}