Scrolling Text

Here is a little script I made and it just vertically scrolls text, but this is about the slowest I can make the text scroll, how could I get it to scroll slower witht his script? I can't increase the timeout else the eye can detect it.

HTML Code:

<script type='text/javascript'>
var done = false;
function scrollUp(x)
{
var box = document.getElementById('inner-scroll-box');
if (Math.abs(x) > box.scrollHeight) done = true;
box.style.top = (x - 1) + 'px'

if (!done) {
setTimeout('scrollUp(' + parseInt(box.style.top) + ')', 25);
} else {
done = false;
scrollUp(100);
}
}
</script>