function externalLinks() {  
  if (!document.getElementsByTagName) return;  
  var anchors = document.getElementsByTagName("a");  
  for (var i=0; i<anchors.length; i++) {  
    var anchor = anchors[i];  
    if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external") {
      anchor.target = "_blank";  
    }
  }  
}  

jQuery(function($){
  externalLinks();
});