A small temporary dam, unless one exists, must be made, so as to secure a still surface. Take two poles, sufficiently long to reach from the bottom of the water to the required line-level. Make a plain mark or notch on both sticks, at a distance from the upper end equal to the distance of the intended line-level above the water, marking that distance in feet and inches. Push the poles down through the water into the earth at the bottom until the notches are both at the level surface of the water, care being taken to have the poles plumb and at a convenient distance apart. Sight across the tops of these two, and set as many more as may be desired to run the line of level to the desired point, and the tops being ranged accurately by the first two, will show a water-level so many feet above that of the water. It is estimated that this is a more accurate way than the use of the ordinary spirit-level.