function handle(delta) {
        if (delta <0)
                zoomOut();
        else
                zoomIn();
}

function wheel(event){
        var delta = 0;
        if (!event) event = window.event;
        if (event.wheelDelta) {
                delta = event.wheelDelta/120;
                if (window.opera) delta = -delta;
        } else if (event.detail) {
                delta = -event.detail/3;
        }
        if (delta)
                handle(delta);
}

if (window.addEventListener)
        window.addEventListener('DOMMouseScroll', wheel, false);
window.onmousewheel = document.onmousewheel = wheel;