var AMOUNT=8; /* distance to scroll for each time */
var TIME=20; /* milliseconds */
var timer=null;

function scrollDown(h)
{	
  if(timer) { clearInterval(timer); timer=null; }  
  if(h)timer=setInterval("scrollIt(true, 'down')",TIME);
}

function scrollUp(h)
{
  if(timer) { clearInterval(timer); timer=null; }
  if(h)timer=setInterval("scrollIt(true, 'up')",TIME);
} 

function scrollIt(h,dtn)
{	
	var direction=h?1:-1;
	var distance=AMOUNT*direction;
	if (dtn=="down") {
		main_content.scrollBy(0,distance);
	}
	else if(dtn=="up") {
		main_content.scrollBy(0,-distance);
	}
}