control apparatus for controlling a windscreen wiper in accordance with the mean moisture level on the windscreen includes a microprocessor 9 arranged to detect via input 22 the time a wiper motor 1 takes to sweep the windscreen . the microprocessor calculates an appropriate delay in dependence on the detected sweep time and then causes a subsequent sweep via transistor 4 . the delay is varied automatically by the microprocessor so as to rapidly clear the windscreen and then stop the wiper .