Element.addMethods({
  scrollTo: function(element, left, top){
	var element = $(element);
	if (arguments.length == 1){
	  var pos = element.cumulativeOffset();
	  window.scrollTo(pos[0], pos[1]);
	} else {
	  element.scrollLeft = left;
	  element.scrollTop  = top;
	}
	return element;
  }
});

Effect.Scroll = Class.create();
Object.extend(Object.extend(Effect.Scroll.prototype, Effect.Base.prototype), {
  initialize: function(element) {
	this.element = $(element);
	if(!this.element) throw(Effect._elementDoesNotExistError);
	this.start(Object.extend({x: 0, y: 0}, arguments[1] || {}));
  },
  setup: function() {
	var scrollOffsets = (this.element == window) 
				? document.viewport.getScrollOffsets() 
				: Element._returnOffset(this.element.scrollLeft, this.element.scrollTop) ;
	this.originalScrollLeft = scrollOffsets.left;
	this.originalScrollTop  = scrollOffsets.top;
  },
  update: function(pos) {
	this.element.scrollTo(Math.round(this.options.x * pos + this.originalScrollLeft), Math.round(this.options.y * pos + this.originalScrollTop));
  }
});