var Marquee = function(id){
    var container = document.getElementById(id),
    slide = container.getElementsByTagName("ul")[0],
    speed = arguments[1] || 10;
    slide.innerHTML += slide.innerHTML;
    var item  = slide.getElementsByTagName("li"),
    critical = item[item.length/2].offsetLeft,//临界值
    delta = 0;
    var rolling = function(){
      delta == -critical ? delta = 0 : delta--;
      slide.style.left = delta + "px";
    }
    var timer = setInterval(rolling,speed)//设置定时器
    container.onmouseover=function() {clearInterval(timer)}//鼠标移到marquee上时，清除定时器，停止滚动
    container.onmouseout=function() {timer=setInterval(rolling,speed)}//鼠标移开时重设定时器
  }