function banner(imgSource,chance) {
this.imgSource = imgSource;
this.chance = chance;
}

function dispBanner() {
with (this) document.write("<IMG SRC='" + imgSource + "' WIDTH=430 HEIGHT=210 ALT=''>");
}
banner.prototype.dispBanner = dispBanner;

banners = new Array();

banners[0] = new banner("images/header1.jpg",
                        10);
banners[1] = new banner("images/header2.jpg",
                        10);
banners[2] = new banner("images/header3.jpg",
                        10);
banners[3] = new banner("images/header4.jpg",
                        10);
banners[4] = new banner("images/header5.jpg",
                        10);
banners[5] = new banner("images/header6.jpg",
                        10);
banners[6] = new banner("images/header7.jpg",
                        10);
banners[7] = new banner("images/header8.jpg",
                        10);
banners[8] = new banner("images/header9.jpg",
                        10);
banners[9] = new banner("images/header10.jpg",
                        10);
banners[10] = new banner("images/header11.jpg",
                        10);
banners[11] = new banner("images/header12.jpg",
                        10);
banners[12] = new banner("images/header13.jpg",
                        10);
banners[13] = new banner("images/header14.jpg",
                        10);
banners[14] = new banner("images/header15.jpg",
                        10);
banners[15] = new banner("images/header16.jpg",
                        10);
banners[16] = new banner("images/header17.jpg",
                        10);                                                                        


sum_of_all_chances = 0;
for (i = 0; i < banners.length; i++) {
sum_of_all_chances += banners[i].chance;
}
function randomBanner() {
chance_limit = 0;
randomly_selected_chance = Math.round((sum_of_all_chances - 1) * Math.random()) + 1;
for (i = 0; i < banners.length; i++) {
chance_limit += banners[i].chance;
if (randomly_selected_chance <= chance_limit) {
document.write("<IMG SRC='" + banners[i].imgSource + "' WIDTH=430 HEIGHT=210 ALT=''>");
return banners[i];
break;
}
}
}