滚动到特定位置时出现4

<script type="text/javascript">
    $(function(){
        var h3_height = $("#tabBox").offset().top;
        $(window).scroll(function(){
            var this_scrollTop = $(this).scrollTop();
            if(this_scrollTop>h3_height ){
                $("#btn").hide();
            }
        });
    });

</script>