A bowling machine is a device that functions as a nets bowler, firing balls at batsmen, providing them with practice. The machine can simulate various types of pace, swing, and spin bowling accurately.
Swing is produced by varying the speeds of the wheels independently using a manual controller - i.e., speeding one wheel up and slowing the other one down, which produces an effect similar to that caused when a football is kicked with the outer part of the foot. The ball swerves away from the faster wheel.
The speed can be varied from 20mph to 95mph.
The bowling machine is most accurate with a round object, and cricket balls are not perfectly round. Moreover, cricket balls are made of leather, which is prone to tearing when squeezed between the spinning wheels. This is why hard polyurethane balls - dimpled for swing - are used.
An advance over the mechanical version is the programmable bowling machine, which can be configured to bowl different types of deliveries in sequence. A programmable bowling machine called Merlyn, which it was claimed could bowl any ball known to man, including Shane Warne's legbreaks, was in the news when it was used by England in the run-up to their successful 2005 Ashes campaign.