How to Scroll Down – JQuery

I’m not a programmer, but I use the code below to scroll the page to the top.

How can I adapt it to make a scroll down?

<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js"></script>

<a class="btnMedio" href="javascript:;">
    <img src="http://desmond.imageshack.us/Himg41/scaled.php?server=41&filename=deixeseuemail.png&res=landing"/>
</a>

<script>
    $('.btnMedio').click(function(){
        $('html, body').animate({scrollTop:1000},'50');
    });
</script>