Why does setInterval seem to have scope issues in my jQuery plugin?

I'm sorry this isn't more specific, but I'm having trouble isolating the issue.

I've written a very simple jQuery plugin that scrolls images or other elements through a div, like a carousel, on a set interval. I wanted this plugin to work with multiple instances on one page, but when I call it on multiple elements, only the last initialized element scrolls. I assume the way I'm using setInterval is the cause, but I don't understand why.

The function for scrolling is as follows, and the full source is linked above.

function scrollRight() {
    // Don't animate if the mouse is over the scrollah
    if (hovering) { return; }

    /* If we're at the end, flip back to the first image
     * 开发者_JAVA技巧before animating, lest we view blankness in the wrapper
     */
    if (position === nChildren) {
        position = 0;
        $wrapper.css('left', '0px');
    }

    // Animate to the next view
    position++;
    $wrapper.animate({
        left: position*-width+'px'
    }, 1000, 'swing', function() {
        // Animation complete.
    });
}
setInterval(scrollRight, 5000);

So why do individual instances of this plugin not scroll once more have been initialized?

I think if you change $wrapper = $this.find('.wrapper'); to var $wrapper = $this.find('.wrapper'); it might work.

Learned this the other day from Stack Overflow: variables that don't use the var keyword are implicitly global in scope, so I think each scroller is overwriting the same global $wrapper variable.

EDIT: might also want to do var $this = $(this);.