function set_tooltip() {
	element      = $('which_download');
    Event.observe(element, "mouseover", function(){
		new Effect.Appear('which_download_box', {'duration':0.5});											 
	});
    Event.observe(element, "mouseout", function(){
		$('which_download_box').hide();											 
	});
 
}
Event.observe(document, 'dom:loaded', set_tooltip, false);