I need to have the page auto scroll down and up then repeat it. My code below is auto scroll down but it didn't back up to the top. Can any one pls help?

<script type="text/javascript">
function pageScroll() {
window.scrollBy(0,50); // horizontal and vertical scroll increments
scrolldelay = setTimeout('pageScroll()',100); // scrolls every 100 milliseconds

<body onLoad="pageScroll()">