关于scrollTop

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>无标题文档</title>
</head><body>
<span id="layerOne">
<table width="539" height="67" border="0" cellpadding="0" cellspacing="0" bgcolor="#000000">
  <tr>
    <td>&nbsp;</td>
  </tr>
</table>
<input type="button" id="Up">
</span>
</body>
</html><script language="JavaScript">
window.Up.onclick=UpTop;
window.Up.value="^"
function UpTop()
{
layerOne.scrollTop=150
alert(layerOne.scrollTop);//为什么会是0，我已经设置了值
}</script>