可滚动div自动滚动

i have a scrollable div on my site but since the content inside is AJAX-driven and is refreshed every second...the div keeps scrolling back up!how do i prevent this? heres the div itself:

$show = '<div class="chatlist" >';
while($row = mysql_fetch_array($sql))
    {
    $messages = $row['messages'];
    $userid = $row['id'];
    $name = $row['name'];
 $show.='
 <span class="nm" >' . $name . ':</span><br/><span class="msg">' . $messages .  '<br/></span>

 ';}
    $show.='</div>';
     echo $show;

heres the css:

<style type="text/css">
    div.chatlist
    {
border: thin dotted #000;
width: 600px;
left:600px;
top:600px;
height:600px;
background-color: #000;
color: #FFF;
border-radius: 5px;
overflow: scroll;   

}
span.nm{
    font-weight: bold;
    text-align:left 
}
span.msg{
text-align: right;
font-style: italic;
 }
    </style>

Also only one CSS rule for both the spans is getting followed.any way around this?

You can set a DIV's scrollTop property using JavaScript:

https://developer.mozilla.org/en/DOM/element.scrollTop

So you would save the value before the content is updated and reinsert it after the update is complete.