Jquery实现缓慢显示下拉菜单

<div class="navMenu">
<ul>
<li class="active"><a href="#">首页</a>
<ul>
<span class="corner"></span>
<li><a href="#">JavaScript+</a></li>
<li><a href="#">语文</a></li>
<li><a href="#">英语</a></li>
</ul>
</li>
<li><a href="#">课程大厅</a></li>
<li><a href="#">学习中心+</a>
<ul>
<span class="corner"></span>
<li><a href="#">JavaScript+</a></li>
<li><a href="#">语文</a></li>
</ul>
</li>
<li><a href="#">帮助</a></li>
</ul>
</div>

*{
padding:0;
margin:0;
}
/*一级菜单*/
.navMenu {
width:570px;
margin:0 auto;
margin-top: 20px;
position: relative;
}
.navMenu ul li{
float: left;
}
li{
list-style: none;
background-color: #eee;
width: 140px;
height: 34px;
text-align: center;
margin-right: 2px;
margin-bottom: 2px;
}
.navMenu ul li:hover{
background:url(images/slide-pannel_14.png) 0 0 repeat-x;
}
ul li a{
line-height: 34px;
text-align: center;
font-size: 20px;
color: #000;
text-decoration: none;
display: block;
padding:0 10px;
}
/*二级菜单*/
.navMenu ul li ul {
display: none;
position:absolute;
top:34px;
}
.active{
background:url(images/slide-pannel_14.png) 0 0 repeat-x;
}
.corner{
display: block;
height: 11px;
background: url("images/bird.png") 61px 0 no-repeat ;
}

<script type="text/javascript" src="js/jquery-1.12.3.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$(".navMenu li").mouseenter(function(){
$(this).find("ul").slideDown("slow"); //慢慢展开
}).mouseleave(function(){
$(this).find("ul").slideUp("slow");//慢慢收起
})
})
</script>