GNU Emacs Manual
P.14 Scrolling With "Wheeled" Mice
Some mice have a "wheel" instead of a third button. You can
usually click the wheel to act as either Mouse-2 or
Mouse-3, depending on the setup. You can also use the wheel to
scroll windows instead of using the scroll bar or keyboard commands.
To do so, turn on Mouse Wheel global minor mode with the command
M-x mouse-wheel-mode or by customizing the option
mouse-wheel-mode. Support for the wheel depends on the system
generating appropriate events for Emacs.
mouse-wheel-scroll-amount determine where and by how much
buffers are scrolled.