jquery ajaxStart不执行

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">

    <head>

        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

        <title>全局Ajax事件应用示例</title>

        <style type="text/css">

            button, div, li, a {

                font-size: 12px;

                font-family: Tahoma;

            }

            

            #loading {

                display: none;

            }

        </style>

        <script type="text/javascript" src="jquery.js">

        </script>

        <script type="text/javascript">

            $(document).ready(function(){

                $("button").click(function(){

                    var url = "http://services.digg.com/stories/top?appkey=http%3A%2F%2Fmashup.com&type=javascript&callback=?";

                    $.getJSON(url, function(data){

                        $.each(data.stories, function(i, story){

                            var content = "<li><a href='" + story.href + "' target='_blank'>" + story.title + "</a>";

                            $("#res ul").append(content);

                            if (i == 3) 

                                return false;

                        });

                    });

                });

                $().ajaxStart(function(){

                    $("#loading").show();

                }).ajaxStop(function(){

                    $(this).hide();

                });

            });

        </script>

    </head>

    <body>

        <button>

            加载数据

        </button>

        <div id="loading">

            数据加载中...<img src="images/ajax-loader.gif" />

        </div>

        <div id="res">

            <ul type="square">

            </ul>

        </div>

    </body>

</html>