We first determined the height of the upper filter barrel above the level of the river. This was done with our surveying instrument, which was set level with the top of the barrel. We sighted with the instrument to a long pole that was held upright at the edge of the water. The pole had been marked off into feet with white chalk marks, and on sighting through the sight holes we found that the hairs came in line with the eleventh chalk mark. The top of the filter was, therefore, 11 feet above the level of the river. Bill figured that it would be necessary to construct a wheel about 15 feet in diameter in order to raise the water to the proper height.

Fig. 250. Surveying for the Water Wheel.