setting document heigth for infinite scrolling ./ infinite scroll not working properly

setting document heigth for infinite scrolling ./ infinite scroll not
working properly

This is my test code which I will use for achieving infinite scrolling.
Problem is that the alert comes up when the scroll is at the top not at
the bottom...
Can any one tell me where I am doing wrong.
The alert box should come up when the I scrolled the full page i.e the
scroll is at the bottom.
I think I need to set document height by any other method but i have no
idea how to do it
Thanks in advance..
$(window).scroll(function () {
if($(window).scrollTop() + $(window).height() == $(document).height() ) {
alert("abc");
}
});