var ft; //初始高度
//--初始化高度--
function init(){
 ft=parseInt(document.documentElement.scrollTop+(document.documentElement.clientHeight-document.getElementById("divMenu").offsetHeight)/6);
 document.getElementById("divMenu").style.top=ft;
}
var flag=0; //只允许move()执行一次的判断变量; 
var speed=5;//移动速度
var acc=100;//加速度
//--移动--
function move(){
 var t=document.getElementById("divMenu").style.top;
 var top=parseInt(t.substring(0,t.length-2));
 var myhigh=parseInt(document.documentElement.scrollTop+(document.documentElement.clientHeight-document.getElementById("divMenu").offsetHeight)/6);
 if(top<myhigh-speed){ //用来缓冲
  document.getElementById("divMenu").style.top=top+speed;
 }else if(top>myhigh+speed){
  document.getElementById("divMenu").style.top=top-speed;
 }
 setTimeout("move()",10);
}  
function scall(){ 
 if(flag==0){
  move();
  flag=1;
 }
} 
window.onscroll=scall; 
window.onload = function() {   
                init();    
            }