document.observe('dom:loaded', function(){
  $$('a.popup').each(function(element){
    new Tip(element, 
      {
        ajax: {
          url: element.rel,
          options: {method: 'get'}
        }
      });
  });
});

document.observe('dom:loaded', function(){
  $$('a.popup_side').each(function(element){
    new Tip(element, 
      {
        ajax: {
          url: element.rel,
          options: {method: 'get'}
        },
        hook : {target: 'bottomLeft', tip : 'topRight'}
      });
  });
});