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Leveling the Instrument One of the most obvious differences between an engineer's level and a self-leveling level is the procedure used in leveling the instrument. Engineer's levels typically have four leveling screws and a spirit level (a sealed vial partially filled with alcohol). A self-leveling level has three leveling screws and a bull's-eye (circular) spirit level. In leveling the four-screw head, the telescope is turned until it is over-two opposite (diagonally opposed) screws. The bubble is approx- imately centered fin the level vial by turning both leveling screws in opposite directions, at the same speed, with both hands. A simple rule is that the bubble moves in the same direction as the left thumb. Fail- ure to move both screws at the same speed will often leave the leveling head wobbly. The procedure is then repeated with the telescope over the other two leveling screws. It is impractical to attempt to exactly center the bubble on the first try, since it will be thrown off during the cross-leveling. Readjusting each pair of screws about three times is usually enough to complete the leveling process. A self-leveling level has a three-screw head and a bull's-eye spirit level. These levels contain a system of internal prisms which allow a level line of sight even if the instrument itself is not exactly level. For a three-screw head, the telescope is aligned over one screw. The telescope is made level by alternately turning this screw, then the other two simultaneously, until the bubble lines up in the center of the bull's-eye. The telescope need not be rotated in the process. Reading the Level Rod The type of rod used in most instream flow work is the self-reading rod, which is read by the levelman as he sights through the telescope and notes the apparent intersection of the horizontal cross hair with the rod. The two most commonly used rods are the wooden, 13-foot Phila- delphia rod, and the 25-foot telescoping fiberglass rod. For extended use, the Philadelphia type rod does not hold up too well under the rigors of river cross-sectioning. However, the wooden rod floats, which can be a definite advantage if the rodman loses his grip. The fiber- glass rod-does not float, and a few have been lost by dropping them into deep pools. The fiberglass rod is very durable, and the 25-foot length eliminates many problems when large variations in elevations are encoun- tered. Regardless of the type of rod used, a standard convention is used in reading them. The graduations on a stadia rod are accurately-painted alternate black and white spaces 0.01 ft (1 cm if metric) in width. The 0.1 and 0.05 ft graduations are emphasized by points or spurs extending the black markings. Tenths are designated by black numerals straddling the proper graduation, whereas whole feet or meters are marked by red numerals. On some rods, small red numerals are placed alongside the -73