26.14 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.
Mouse wheel support only works if the system generates appropriate
events; whenever possible, it is turned on by default. To toggle this
feature, use M-x mouse-wheel-mode.
mouse-wheel-scroll-amount determine where and by how much
buffers are scrolled. The variable
mouse-wheel-progressive-speed determines whether the scroll
speed is linked to how fast you move the wheel.