我想实现个鼠标按下滚动速度加快


<style>
body{ padding:0; margin:0;}
img{ width:100px; height:100px;}
.up,.down{ width:100px; height:20px; background:#9CC; text-align:center;}
</style>
<div class="up" id="up">向上</div>
<div id="scrollup" style=" width:100px; height:100px; overflow:hidden">
<div id="scrollup1">
     <img src="image/person1.jpg" />
        <img src="image/person2.jpg" />
        <img src="image/person3.jpg" />
        <img src="image/person4.jpg" />
    </div>
    <div id="scrollup2"></div>
</div>
<div class="down" id="down">向下</div>
<script>
var speed = 10;
var scrollup= document.getElementById("scrollup");
var scrollup1 = document.getElementById("scrollup1");
var scrollup2 = document.getElementById("scrollup2");
scrollup2.innerHTML = scrollup1.innerHTML;
function Marqueeup(){
if(scrollup2.offsetTop == scrollup.scrollTop ){
scrollup.scrollTop -= scrollup2.offsetHeight;
}else{
scrollup.scrollTop++;
}
}
function Marqueedown(){
if( scrollup.scrollTop ==0 ){
scrollup.scrollTop += scrollup2.offsetHeight;
}else{
scrollup.scrollTop--;
}
}
var mar = setInterval(Marqueeup,speed);var up = document.getElementById("up");
var down = document.getElementById("down");
up.onclick = function(){
clearInterval(mar);
mar = setInterval(Marqueeup,speed);
}
down.onclick = function(){
clearInterval(mar);
mar = setInterval(Marqueedown,speed);
}</script>现在我实现了鼠标点击上下时 图片上下滚动。 但是我还想实现当鼠标左键按下时滚动速度变快那种效果。。求大神帮忙