function preloadImages(){
        if(typeof(document.pics) == 'undefined'){
          document.pics= new Array();
        }
              
        len = arguments.length;
        for(i = 0; i<len; i++) {
          document.pics[i] = new Image();
          document.pics[i].src = arguments[i];
        }
      }
        
      function img_swap(imgName,newSrcIndex)    
      {
        if(document.images) {
          document[imgName].src = document.pics[newSrcIndex].src;
        }
      }
             
      preloadImages( "/mathnight/image/strip/guide.gif", 
                     "/mathnight/image/strip/guide_over.gif",
                     "/mathnight/image/strip/activities.gif",
                     "/mathnight/image/strip/activities_over.gif",
                     "/mathnight/image/strip/supplements.gif",
                     "/mathnight/image/strip/supplements_over.gif",
                     "/mathnight/image/strip/links.gif",
                     "/mathnight/image/strip/links_over.gif",
                     "/mathnight/image/strip/schools.gif",
                     "/mathnight/image/strip/schools_over.gif");