var xspan     = 15; 
var xoffset   = 50; 
var xpos      = 0; 
var direction = 'right'; 
var shaking   = false; 
var reference; 
 
function shake(){ 
  if(shaking){ 
    if(direction == 'right'){ 
      xpos++; 
      getElement('id','shake',null).style.left = xoffset+xpos+'px'; 
      if(xpos == xspan) direction = 'left'; 
    }else{ 
      xpos--; 
      getElement('id','shake',null).style.left = xoffset+xpos+'px'; 
      if(xpos == 0) direction = 'right'; 
    } 
  } 
} 
 
function stopShake(){ 
  shaking = false; 
  window.clearInterval(reference); 
} 
 
 
 
function startShake(){ 
  shake(); 
  reference = window.setInterval("shake()",50); 
  if(!shaking){ 
    shaking = true; 
    shake(); 
  } 
}