Horizontal scroll with mouse in a div not smooth

Hi this code is working but my scroll isn't smooth how can i do?

(function() {
  function scrollHorizontally(e) {
    e = window.event || e;
    var delta = Math.max(-1, Math.min(1, (e.wheelDelta || -e.detail)));
    document.getElementById('mydiv').scrollLeft -= (delta * 40); // Multiplied by 40
    e.preventDefault();
  }
  if (document.getElementById('mydiv').addEventListener) {
    // IE9, Chrome, Safari, Opera
    document.getElementById('mydiv').addEventListener("mousewheel", scrollHorizontally, false);
    // Firefox
    document.getElementById('mydiv').addEventListener("DOMMouseScroll", scrollHorizontally, false);
  } else {
    // IE 6/7/8
    document.getElementById('mydiv').attachEvent("onmousewheel", scrollHorizontally);
  }
})();