var sLenght = 0;
  
function move_up(){

	sNext = setTimeout("move_up()",20);
	sLenght = sLenght-4;
	if (sLenght < 0){ sLenght = 0; }

	document.getElementById('scroll_chat').scrollTop = sLenght;


}

function move_down(){

	sNext = setTimeout("move_down()",20);
	sLenght = sLenght+4;

	document.getElementById('scroll_chat').scrollTop = sLenght;

}