Marquee滚动到最后一个就不循环了？撒子原因

Question

<div id="l">
<div class="nav">公告事宜</div>
<div id="demo" class="ay scroll l1">
<div id="demo1">
<ul>
<asp:literal runat="server" id="l52"></asp:literal>
</ul>
</div>
<div id="demo2"></div>
</div>
<script language="JavaScript" type="text/javascript">
var speed=50
demo2.innerHTML=demo1.innerHTML
function Marquee(){
if(demo2.offsetTop-demo.scrollTop<=0)
demo.scrollTop-=demo1.offsetHeight
else{
demo.scrollTop++
}
}
var MyMar=setInterval(Marquee,speed)
demo.onmouseover=function() {clearInterval(MyMar)}
demo.onmouseout=function() {MyMar=setInterval(Marquee,speed)}
</script>