Explanation

Governor is the device used to control the speed of engines when there is variation in the load

speed of an engine varies in two ways

1. during each revolution or cyclic variation 
2. over a number of revolutions

during each revolution or cyclic variation the speed of the engine varies due to variation in output  torque of the engine

over a number of revolution, the speed of the engine varies due to variation in load upon engine and the speed maintained by mounting a governor