Event.observe(document, 'contentloaded', function() {
  var posts = new Array;
  $$('.hentry').each(function(item){
      posts.push(item);
  });
  
  var i = 0;

  Event.observe(document, 'keypress', function(event){ 
  if (i < posts.length  &&  i >= 0 ){
     if(event.keyCode == '74'){
       posts[i].scrollTo();
       i++;
     }
     if(event.keyCode == '75'){
       posts[i].scrollTo();
       i--;
     }
   }
  });
});