slideDown not working or working like .show

Question

I am trying to slideDown() a hidden div but it only show() it without the slideDown() effect. when i clock on button content apears suddenly after 2-3 seconds here is js

var hash = window.location.hash.substr(1);
var href = $('#nav li a').each(function(){
    var href = $(this).attr('href');
    if(hash==href.substr(0,href.length-5)){
        var toLoad = hash+'.html #content';
        $('#content').load(toLoad)
    }                                           
});

$('#nav li a').click(function(){

    var toLoad = $(this).attr('href')+' #content_id1';
    $('#content').fadeOut(1000,loadContent);
    window.location.hash = $(this).attr('href').substr(0,$(this).attr('href').length-5);
    function loadContent() {
        $('#content').load(toLoad,'',showNewContent())
    }
    function showNewContent() {
        $('#content').slideDown('normal');


    }
    return false;       
});

here is css

#content {width:934px;margin-top:238px;position:relative;height:500px;display:none;}
#content #content_id1{width:934px; background-color:white; height:500px; position:relative;top:4px;margin-top:238px;margin-bottom:50px;padding:20px;}

and here is html

            <ul id="nav">
                <li>
                  <a href="products/rania.php"><img src="img/products/1.jpg"/></a>
                </li>
                <li class="under">
                 <a href="products/lutron.php"><img src="img/products/2.jpg"/></a>
                </li>

            </ul>

            <div id="content">   </div>

Answer 1

Read jQuery's API about slideDown(). Where is no parameter as normal, duration should be numeric value or slow, or fast.