var stop=true;
$(window).scroll(function(){
    totalheight = parseFloat($(window).height()) + parseFloat($(window).scrollTop());
    if($(document).height() <= totalheight){
        if(stop==true){
            stop=false;
            $.post("ajax.php", {start:1, n:50},function(txt){
                $("#Loading").before(txt);
                stop=true;
            },"text");
        }
    }
});

<div id="Loading">Loading...</div>