var scrollIntervalId = null;

function stopScroll() {
  if (scrollIntervalId) {
    clearInterval(scrollIntervalId);
    scrollIntervalId = null;
  }
}

function startScroll(id, speed, interval) {
  var div = document.getElementById(id);

  div.scrollTop+=speed;

  if (scrollIntervalId == null)
    scrollIntervalId = setInterval('startScroll("' + id + '", ' + speed + ');', interval);
}

function scroll(id) {
  var whereto = 0 - (event.wheelDelta / 5);
  startScroll('chapter_contents', whereto, 30);
  stopScroll();
}