jquery on scroll down

Check current scrollTop vs previous scrollTop

var lastScrollTop = 0;
$(window).scroll(function(event){
   var st = $(this).scrollTop();
   if (st > lastScrollTop){
       // downscroll code
   } else {
      // upscroll code
   }
   lastScrollTop = st;
});

Syntax
Trigger the scroll event for the selected elements:
$(selector).scroll()

Attach a function to the scroll event:
$(selector).scroll(function)

$("#main-div" ).on( 'wheel', function() {
   alert("Hello! I am an alert box!!");
});