var d = document;
onload = function() {
   var viewport_height = (self.innerHeight) ? self.innerHeight : (document.documentElement && document.documentElement.clientHeight) ? document.documentElement.clientHeight : (document.body) ? document.body.clientHeight : 0;
   var page_height = d.getElementsByTagName('body')[0].offsetHeight;
   var ttl = d.getElementById('to_top_link');
   if (page_height > viewport_height) ttl.style.visibility = 'visible';
};