onClipEvent (load) {

    var Xdirection = 1; 

}


onClipEvent (enterFrame) {

    // the variable changes the direction

    _x = _x + 10*Xdirection;



    // if the instance goes too far

    // in either direction,

    // change the SIGN of the variable


    if(_x > 300){

        Xdirection = -1;

    } 


    if (_x < 0){

        Xdirection = 1;

    } 




}