如何做滚动效果？

1<script>var timer</script>

1<div id="demo" style="width:100;height:100;overflow:auto">   
2就是在最中间有个层或话框，有上下两个图片，把鼠标放到向上的图片时向上滚动，向下是向下滚动。   
3要怎么实现？把代码帖出来行吗？   
4</div>

1<img onmouseout="clearInterval(timer)" onmouseover="timer=setInterval('demo.scrollTop--',1)" src="up.gif"/>

1<img onmouseout="clearInterval(timer)" onmouseover="timer=setInterval('demo.scrollTop++',1)" src="down.gif"/>

1<tr>
2<td height="49"><img alt="鼠标点住不放加速移动" class="opacity" height="31" onmousedown="movover();movstar(-3,2)" onmouseout="movover();o_up(this)" onmouseover="movstar(-1,20);o_down(this)" onmouseup="movover();movstar(-1,20)" src="image/up.gif" width="10"/>
3</td>
4</tr>

1<tr>
2<td valign="top"><img alt="鼠标点住不放加速移动" class="opacity" height="31" onmousedown="movover();movstar(3,2)" onmouseout="movover();o_up(this)" onmouseover="movstar(1,20);o_down(this)" onmouseup="movover();movstar(1,20)" src="image/down.gif" width="10"/></td>
3</tr>

1<tr></tr>