// 滚动到底部，向下的箭头消失
        var $down = $('.down');
        var $window = $(window);
        var $document = $(document);
        $window.scroll(function(){
            if ($document.scrollTop() + $window.height() >= $document.height()) {
                $down.hide();
            } else {
                $down.show();
            }
        });