The 255-210 Stillwell with Fixed Point consists of a pointed rod called the fixed point mounted in a tube called the stillwell (or stilling well). It is placed inside the evaporation pan, one foot from the north edge. The fixed point is attached to the center of the base inside the stillwell. The point is 7.5 inches above the bottom of the evaporation pan when in position.
The stillwell makes readings more precise by eliminating wind-caused surges and ripples in the water level. It has two small openings located opposite each other near the base to permit the flow of water in and out of the well.