I'm adding a function to my site, i want to allow the user to change the scrolldelay of a marquee

here is the function:
function changespeed()
var speed = prompt("Enter your desired scrolling speed, normal is 450");
document.thelist.scrolldelay = speed;

and here is the marquee opening tag:
<marquee attributes name="thelist" id="thelist" text="white" bgcolor= "black" width="550" height="55" direction="up" scrolldelay="450">
I've been searching but I can't find how to reference the scrolldelay
If anyone could help i'd appreciate it