一个滚动广告的问题？

Question

<div id=AdLayer style='position:absolute; width:61px; height:59px; z-index:120;  left: 800px; top: 200px' ><a href="http://www.5dmedia.com/bbs" > <img src='/images/ad01.gif' border="0" > </a > 
</div >
<div id=AdLayer1 style='position:absolute; width:61px; height:59px; z-index:120;  left: 800px; top: 200px' ><a href="http://www.5dmedia.com/bbs22" > <img src='/images/ad01.gif' border="0" > </a> 
</div >
<SCRIPT LANGUAGE="JavaScript" >
<!--
function initAd() {
document.all.AdLayer.style.posTop = -100;//设置onLoad事件激发以后，广告层相对于固定后的y方向位置
document.all.AdLayer.style.visibility = 'visible'//设置层为可见
MoveLayer('AdLayer');//调用函数MoveLayer()
}
function MoveLayer(layerName) {
var x = 500;//浮动广告层固定于浏览器的x方向位置
var y = 50;//浮动广告层固定于浏览器的y方向位置
var diff = (document.body.scrollTop + y - document.all.AdLayer.style.posTop)*.40;
var y = document.body.scrollTop + y - diff;
eval("document.all." + layerName + ".style.posTop = y");
eval("document.all." + layerName + ".style.posLeft = x");//移动广告层
setTimeout("MoveLayer('AdLayer');", 20);//设置20毫秒后再调用函数MoveLayer()
}function initAdd() {
document.all.AdLayer1.style.posTop = -200;//设置onLoad事件激发以后，广告层相对于固定后的y方向位置
document.all.AdLayer1.style.visibility = 'visible'//设置层为可见
MoveLayer1('AdLayer1');//调用函数MoveLayer()
}
function MoveLayer1(layerName1) {
var x = 5;//浮动广告层固定于浏览器的x方向位置
var y = 50;//浮动广告层固定于浏览器的y方向位置
var diff = (document.body.scrollTop + y - document.all.AdLayer1.style.posTop)*.40;
var y = document.body.scrollTop + y - diff;
eval("document.all." + layerName1 + ".style.posTop = y");
eval("document.all." + layerName1 + ".style.posLeft = x");//移动广告层
setTimeout("MoveLayer1('AdLayer1');", 20);//设置20毫秒后再调用函数MoveLayer()
}
//-- >
</script>
<SCRIPT LANGUAGE="JavaScript" FOR=window EVENT=onload >
<!--
initAd();//载入页面后，调用函数initAd()
initAdd();//载入页面后，调用函数initAd()
</SCRIPT>
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