Which mouse control will zoom in or zoom out in Google Earth?

In google Earth the keyboard controls for Zoom are as follows:

Zoom In: + (plus) key, PgUp key

Zoom Out: - (minus) key (both keyboard and numpad), PgDn key

Tip: to use the 'Page Down' key, make sure 'Num Lock' on your keyboard is off.

Also, the Scroll wheel on the mouse can zoom in and out.