jquery-1.4 v. jquery-1.4.2 -- what am I missing??

jquery-1.4 v. jquery-1.4.2 -- what am I missing??

Very first day - so "newbie" in every sense of the word.

I downloaded jquery-1.4.2.js and attempted to run the example I found at: http://www.w3schools.com/jquery/default.asp (see code sample below)

I get the following error:
$ is not defined

Browser: firefox

I looked at the sample code and determined they are using jquery-1.4.js so I downloaded that.

if I go back into my sample code and change to jquery-1.4.js
The sample code works just fine

***** Sample Code *****
<html>

<head>
<script type="text/javascript" src="js/jquery-1.4.js"></script>
<script type="text/javascript">
$(document).ready(function(){
  $("p").click(function(){
  $(this).hide();
  });
});
</script>
</head>

<body>
<p>If you click on me, I will disappear.</p>
<p>If you click on me, I will disappear.1</p>
<p>If you click on me, I will disappear.2</p>
<p>If you click on me, I will disappear.3</p>
<p>If you click on me, I will disappear.4</p>
</body>

</html>