未捕获的引用错误:未定义 ajax (重复 2 次）

我有这个表格，我必须用Ajax处理。

<script type="text/javascript" src="http://code.jquery.com/jquery-1.9.1.js"></script>
<div class="column1">
    <form class="form box" action="javascript:networkCheck();" method="post" enctype="application/x-www-form-urlencoded">
        <h2>Fly!!</h2>
        <table width="100%" id="fp_network_choice">
            <tr><td style="border-bottom: 1px dashed #FFF;">Choose Network</td>
            <td style="border-bottom: 1px dashed #FFF;"></td>
            </tr>
            <tr>
            <td><input type="radio" name="network" id="vatsim" value="vatsim"> VATSIM</td>
            <td><input type="radio" name="network" id="ivao" value="ivao"> IVAO</td>
            </tr>
            </tbody></table>
        <div class="info" style="display:none" id="loading">Verifying status and flight plan, please wait...</div>
        <div class="info" style="display:none" id="error">Goal</div>
        <br><tr><td><input onclick="networkCheck()" id="check" type="submit" value="Check"></td></tr>
    </form>
</div>
<script>

        function networkCheck(){
            var vatsim = 1;
            var icao = 0;
            ajax.open("POST","network_check.php",true);
            ajax.onreadystatechange=function(){
                if(ajax.readyState==4)
                {
                document.getElementById('error').style.display = 'block';
                }
            }
        ajax.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
        ajax.send("vatsim="+vatsim+"&ivao="+ivao);
        document.getElementById('loading').style.display = 'block';
        }
</script>
</head>

我收到此错误未捕获的引用错误：未定义 ajax （重复 2 次）。我看到chrome调试器的错误。

我不知道为什么？也许是JQuery版本???