var scrollDirection = 10;
function pageScroll() {
    window.scrollBy(0,scrollDirection); // horizontal and vertical scroll increments
    scrolldelay = setTimeout('pageScroll()',5); // scrolls every 50 milliseconds
    if ( (window.scrollY === 0) || (window.innerHeight + window.scrollY) >= document.body.offsetHeight) {
        scrollDirection = -1*scrollDirection;
    }
}
pageScroll();