jQuery设置div一直在页面顶部显示的方法

<script src="JS/jQuery-1.8.3.js" type="text/javascript"></scrIPt>
<script type="text/JavaScript">
$(function () {
var elm = $('#pordattr');
var startPos = $(elm).offset().top;
$.event.add(window, "scroll", function () {
var p = $(window).scrollTop();
$(elm).CSS('position', ((p) > startPos) ? 'fixed' : 'static');
$(elm).css('top', ((p) > startPos) ? '0px' : '');
});
});
</script>

<div id="porDAttr">
<input type="text" value="奇异果" id="txtSearchImage" runat="server" />
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Button" />
</div>