[Aetae]

var start = function(){               
        var newTop;
        var objHeight = obj.offsetHeight;
        var top = obj.offsetTop;
        var fast = (obj.fast) ? 2 : 1;
        if(obj.down){       
            newTop = ((objHeight+top) > height) ? top-(speed*fast) : top;   
            obj.style.top = newTop + "px";
        }; 
        if(obj.up){     
            newTop = (top < 0) ? top+(speed*fast) : top;
            obj.style.top = newTop + "px";
        };
    }; 
    obj.interval = setInterval(start, 50);