var drop = {

   init: function(){
     $$('#nav_bar>ul>li').each(function(el){
        el.addEvent('mouseover', function(){
										 
            var ul = el.getElement('ul');
            if( $chk(ul) ){
                ul.set('tween', {duration:250});
                ul.tween('opacity', 1);
            }
        });
        el.addEvent('mouseout', function(){
										 
              var ul = el.getElement('ul');
            if( $chk(ul) ){
                ul.set('tween', {duration:500});
                ul.tween('opacity', 0);
            }
        });         
    });
  }
}



window.addEvent('domready', drop.init);