title: How to detect vertical scroll position using jquery?
body: <p>Using jquery, how can you set an event that triggers when there is a  vertical scroll visible, and when you change from top half of page to bottom half of page, and vice versa.</p>
  
  <p>So for example, if the vertical scroll bar is there, and then I am looking somewhere on the page which is in the top half of the page, and then move down so I am in the bottom half of the page, the function happens. Then if I move back up so I am in the top half again, the function happens.</p>
  
  <p>Thanks.</p>