对联式广告问题?

请问移动滚动条时 如何可以使"广告"到页面底部(左边)就停止向下(向左)无限延伸...这是我的代码:
ad.jsvar delta=0.15
var collection;
document.ns = navigator.appName == "Microsoft Internet Explorer"<!--ubbcodetab-->    <!--/ubbcodetab-->
function floaters() {
this.items<!--ubbcodetab-->    <!--/ubbcodetab-->= [];
this.addItem = function(id,x,y,content)
{
document.write('<DIV id='+id+' style="Z-INDEX: 10; POSITION: absolute; width:80px; height:60px;left:'+(typeof(x)=='string'?eval(x):x)+';top:'+(typeof(y)=='string'?eval(y):y)+'">'+content+'</DIV>');<!--ubbcodetab-->    <!--/ubbcodetab--><!--ubbcodetab-->    <!--/ubbcodetab--><!--ubbcodetab-->    <!--/ubbcodetab--><!--ubbcodetab-->    <!--/ubbcodetab--><!--ubbcodetab-->    <!--/ubbcodetab-->
var newItem= {};
newItem.object = document.getElementById(id);
newItem.x = x;
newItem.y = y;this.items[this.items.length] = newItem;
}
this.play = function()
{
collection = this.items
setInterval('play()',10);
}
}
function play()
{if(screen.width<800 || (!document.ns))
{
for(var i=0;i<collection.length;i++)
{
collection[i].object.style.display<!--ubbcodetab-->    <!--/ubbcodetab-->= 'none';
collection[i].object.style.visibility ='visible';
}
return;
}
for(var i=0;i<collection.length;i++)
{
var followObj= collection[i].object;
var followObj_x= (typeof(collection[i].x)=='string'?eval(collection[i].x):collection[i].x);
var followObj_y= (typeof(collection[i].y)=='string'?eval(collection[i].y):collection[i].y);if(followObj.offsetLeft!=(document.body.scrollLeft+followObj_x)) {
var dx=(document.body.scrollLeft+followObj_x-followObj.offsetLeft)*delta;
dx=(dx>0?1:-1)*Math.ceil(Math.abs(dx));
followObj.style.left=followObj.offsetLeft+dx;
}if(followObj.offsetTop!=(document.body.scrollTop+followObj_y)) {
var dy=(document.body.scrollTop+followObj_y-followObj.offsetTop)*delta;
dy=(dy>0?1:-1)*Math.ceil(Math.abs(dy));
followObj.style.top=followObj.offsetTop+dy;
}
followObj.style.display<!--ubbcodetab-->    <!--/ubbcodetab-->= '';
}
}<!--ubbcodetab-->    <!--/ubbcodetab-->
var theFloaters= new floaters();
var followSwfRight = 'http://www.linkwan.com/gb/images/ads/coupletr.swf';
var followSwfLeft = 'http://www.linkwan.com/gb/images/ads/coupletr.swf';
theFloaters.addItem('followDiv1','document.body.clientWidth + 75',315,'<object classid=clsid:D27CDB6E-AE6D-11cf-96B8-444553540000 codebase=http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,29,0 width=110 height=300 id=linkwanAD><param name=movie value='+followSwfLeft+'><param name=quality value=high><embed src=coupletl.swf quality=high pluginspage=http://www.macromedia.com/go/getflashplayer type=application/x-shockwave-flash width=110 height=300></embed></object>');
<!--ubbcodetab-->    <!--/ubbcodetab-->theFloaters.addItem('followDiv2',1,315,'<object classid=clsid:D27CDB6E-AE6D-11cf-96B8-444553540000 codebase=http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,29,0 width=110 height=300 id=linkwanAD><param name=movie value='+followSwfRight+'><param name=quality value=high><embed src=coupletr.swf quality=high pluginspage=http://www.macromedia.com/go/getflashplayer type=application/x-shockwave-flash width=110 height=300></embed></object>');
<!--ubbcodetab-->    <!--/ubbcodetab-->theFloaters.play();
这是html调用代码:<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>对联AD</title>
</head><body><script language=JavaScript event=FSCommand() for=linkwanAD>
followDiv1.style.visibility='hidden';
followDiv2.style.visibility='hidden';
</script>
<script language=JavaScript src="ad.js"></script><p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p>对联AD</p>
<p></p>
</body>
</html>