var lastScrollY=0; 
function heartBeat(){ 
var diffY = document.documentElement.scrollTop; 
var backi = document.getElementById("backi");
var percent=.1*(diffY-lastScrollY); 
if(percent>0)percent=Math.ceil(percent); 
else percent=Math.floor(percent); 
backi.style.pixelTop+=percent; 
lastScrollY=lastScrollY+percent; 
} 

if (!document.layers) {window.setInterval("heartBeat()",1); }