Why is jquery "not defined"?

Why is jquery "not defined"?

I have already tried to get going with two other tutorials, this is my third attempt.

I have downloaded the developer version of jquery-1.9.0.

It`s in a folder on the desktop along with my several attempts to get going

This is my latest effort...

<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Demo</title>
  </head>
  <body>
    <a href="http://jquery.com/">jQuery</a>
    <script src="jquery.js"></script>
    <script>
      var foo = "bar";

      $( document ).ready(function() {

        $("a").click(function( event ) {

          alert("Thanks for visiting!");

        });

});
    </script>
  </body>
</html>

All I seem to get regarding jquery is some version of the message (in Firebug)...
"ReferenceError: jquery is not defined" .
For the above code, "$" is "not defined"

Does anyone have any ideas about what I`m doing wrong?