<style>
#messages
{
width:300px;
height:150px;
overflow:auto;
border:1px solid silver;
}
</style>
<script type="text/javascript" src="/jquery-3.3.1.min.js"></script>
<script>

function send()
{

var mess=$("#mess_to_send").val();

$.ajax({
type: "POST",
url: "add_mess.php",
data:"mess="+mess,

success: function(html)
{
load_messes();

$("#mess_to_send").val('');
}
});
}


function load_messes()
{
$.ajax({
type: "POST",
url: "load_messes.php",
data: "req=ok",

success: function(html)
{

$("#messages").empty();

$("#messages").append(html);

$("#messages").scrollTop(90000);
}
});
}
</script>

<table>
<tr>
<td>
<div id="messages">
</div>
</td>
</tr>
<tr>
<td>
<form action="javascript:send();">
<input type="text" id="mess_to_send">
<input type="button" value="Отправить.">
</form>
</td>
</tr>
</table>

<script>
load_messes();
setInterval(load_messes,3000);
</script>





Просто не срабатывает самом начале.