function listUpdate(number){
	
	var current_position=$('list_libri').getStyle('top').toInt();
	var new_position=$('list_libri').getStyle('top').toInt();
	
	if (number==1 && current_position < 0){
		new_position=$('list_libri').getStyle('top').toInt()+254;
		$('frDown').setStyle('display','block');
	}
	
	if (number==-1 && current_position > - ($('list_libri').getHeight()-254)){
		new_position=$('list_libri').getStyle('top').toInt()-254;
		$('frUp').setStyle('display','block');
	}
	var height=$('list_libri').getHeight();
	if ((new_position - 254) <  - (height-254)){
		$('frDown').setStyle('display','none');
		$('frUp').setStyle('display','block');
	}
	if ((new_position + 254) >=  0){
		$('frUp').setStyle('display','none');
		$('frDown').setStyle('display','block');
	}
	if ((new_position + 254) <  0){
		$('frUp').setStyle('display','block');
		$('frDown').setStyle('display','none');
	}

	var myEffect = new Fx.Morph('list_libri', {duration: 'long', transition: Fx.Transitions.Sine.easeOut});
	myEffect.start({'top': [current_position, new_position]});
}

if ($('list_libri').getHeight()-254 < 0){
	$('frDown').setStyle('display','none');
	$('scorri').setStyle('display','none');
}