I'm using this code to make my navbar change to a fixed navbar when the user scrolls down a bit. How can I make the animation slide up instead of slide down? The original fiddle can be found here <http://jsfiddle.net/CZ87p/132/>.

```
var nav = $('.nav');
var scrolled = false;

$(window).scroll(function () {

    if (20 < $(window).scrollTop() && !scrolled) {
        nav.addClass('visible').animate({ top: '0px' });
        scrolled = true;
    }

   if (20 > $(window).scrollTop() && scrolled) {
        nav.removeClass('visible').animate('top', '-30px');
        scrolled = false;      
    }
});

```