How do I find the offset of the bottom of my screen?  Suppose I sometimes display a DIV with a high z-index that overlays part of my screen.  And suppose sometimes part of this DIV is too low, and so can't be seen.  I want to scroll the main page lower, so that the overlaying div can be seen in its entirety.

To do that, I need to know 2 numbers - the current y-position of the bottom of the main page, and the current (hidden) y-position of the bottom of the overlaying div. 

What properties would I use and what objects would I use?

Thanks.