A sprinkler for irrigating a square or other polygonally-shaped land area with the sprinkler including a unique structure which will accurately distribute an equal quantity of water on each increment of land area being irrigated. The sprinkler includes a rotatable nozzle in which the speed of the nozzle is varied by a gear drive mechanism which receives its power from a water powered turbine device. The rotational speed of the nozzle is slower when irrigating corner area of the land area being irrigated and faster when irrigating intermediate straight-sided portions of the land area with the nozzle including a diffuser which becomes operative to reduce the length of trajectory of the water being discharged when the sprinkler is irrigating the intermediate straight-sided portions of the land area being irrigated.