[jQuery] How to center div horizontally + vertically?

[Ryura]

Hi everyone,

I'm wondering the best way to center a div horizontally and vertically
on the fly.

I have:
jQuery("a").click(function () {
                            jQuery("#box")./*verticalCenter().*/
add("#loading").show();
});

Basically I just want #box to be centered on the screen and then
shown. I'm clueless as to how to do this, however. I tried using an
excerpt from an older version of the Dimensions plugin that I found on
here, but it seems to be outdated now (it doesn't work in Safari).

Thanks!