date picker javascript not working

JavaScript
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<link rel="stylesheet" type="text/css" href="style.css" media="screen" />
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.9/jquery-ui.js"></script>
<script>
$(function() {
    $( "#datepicker" ).datepicker();
});
</script>
</head>
<body>
    <div class="demo">
    <p>Date: <input type="text" id="datepicker"></p>
    </div><!-- End demo -->
</body>
</HTML>
Source

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